Unavenger
2019-08-03, 04:23 PM
Introduction
So, in 3.5, Pathfinder and 5th edition, there's no difference between being at 1 hit point and being at full hit points, except for how much more damage you have to take to knock you to 0 (or -1 in 3.P, -10 in 3.5, or -CON in PF). The exceptions, mainly Power Words, are not particularly common or commonly-used (notwithstanding the monstrously-overpowered PW: Pain, which deals... hit point damage). This may not be a problem if you're not angling for a particularly realistic game, but it can shake suspension of disbelief when your worn-down fighter who'll be knocked out by the next punch that lands is leaping, bounding and swinging away the same way he was before he took seventeen greatswords to the face (replace with "mitigated the damage from seventeen greatswords by spending hit points" or "spent hit points to avoid getting hit by seventeen greatswords" to taste, depending on how you see hit points).
I'm sure you can use these house rules for any edition, or even any game which has levels and in which you gain hit points as you level up.
The Rule in Brief
The hit points you gain at a particular level are specific to that level. When you run out of hit points for a level, you lose some of the benefits of that level. That's it! You, the DM using this rule, can decide with your group how much goes away when you lose a level (it gets a little messy with spell slots), but the important part is that you shouldn't make people lose hit points because they lost hit points.
Non-Specific Example
Let's consider a class table, from a simple system I've made up for this example:
Level
Hit Points
Verbosity
Grandeur
Mana Per Day
1
6+healthiness
2+truthiness
3
4
2
9+2*healthiness
3+truthiness
5
7
3
12+3*healthiness
4+truthiness
7
10
Suppose a character is third level, and has a healthiness of 2 and a truthiness of 3. They don't just mark their hit points as 18, their verbosity as 7, their grandeur also as 7, and their mana per day as 10. Instead, they write it out in a table, similar to the class table above (the italics here represent pencil):
Level
Hit Points
Verbosity
Grandeur
Mana
1
8/8
5
3
4/4
2
5/5
1
2
3/3
3
5/5
1
2
3/3
Total
18/18
7/7
7/7
10/10/10
The table now shows the increases gained at each level. It also shows the total of all the values. Because Verbosity and Grandeur aren't a resource you can spend (they're like THAC0, BAB, Proficiency bonus, or your preferred way of determining how good you are at hitting things) they'll only go down when you lose levels. Mana points can be lost either from using them up, or losing them when you lose levels. So, there are three values: the current value ("How much mana do I have right now?", the current maximum "What's my maximum mana value at the current level, given my hit point total"), and the actual maximum ("What would be the maximum amount of mana I could have, at my full level, if my hit point total were full?"). For hit points, you don't bother with the current maximum because we assume you can be healed back up a level, so the current maximum isn't relevant. Now, let's start off by supposing the character loses 7 hit points, and therefore loses a level:
Level
Hit Points
Verbosity
Grandeur
Mana
1
8/8
5
3
4/4
2
3/5
1
2
3/3
3
0/5
1
2
3/3
Total
11/18
6/7
5/7
7/7/10
Let's look at the total line, and the verbosity stat to start off. Three points of verbosity come from your truthiness score, two of them from first level, and one from second level. That makes six at second level. Sure enough, you're down to second level, and down to six verbosity. Another way of looking at it is that you had seven verbosity, one of which came from level 3. You've lost level 3, so now you're down to six verbosity. You lose 2 points of grandeur the same way. Your current and maximum mana similarly flop 3 points apiece. The hit points have gone down by 7 - nothing unusual there.
Now, let's look at the hit points on each level. Your level 3 hit points are all gone, and your level 2 hit points are down by 2. So far, so good. Let's cast a 2 mana spell in revenge, smiting our foe:
Level
Hit Points
Verbosity
Grandeur
Mana
1
8/8
5
3
4/4
2
3/5
1
2
1/3
3
0/5
1
2
3/3
Total
11/18
6/7
5/7
5/7/10
Previously, we had 7/7/10 mana. Now, it's 5/7/10. If you look at the 2nd-level and 3rd-level rows, you'll see that we can't access the 3 mana we gain at 3rd level, because we're not at 3rd level. We use the 2nd-level mana instead. However, if we'd got the first hit in, the table would look like this:
Level
Hit Points
Verbosity
Grandeur
Mana
1
8/8
5
3
4/4
2
3/5
1
2
3/3
3
0/5
1
2
1/3
Total
11/18
6/7
5/7
7/7/10
We lost 2 mana from the third level when we cast the spell. We then lost all our current mana and current maximum mana from level 3. That means that we lose 1 current, and 3 current maximum mana. They both, therefore, drop to 7.
5e Example: Super-Simple Fighter
Level
Hit Points
Proficiency
1-4
48/48
+2
5-8
44/44
+1
9-12
44/44
+1
13-16
32/44
+1
17
0/11
+1
Total
168/191
+5/+6
This is a relatively easy start. The 17th-level fighter here has actually lost two levels, but from this table, you can't tell because it doesn't matter. All that matters is how many hit points apply to each point of proficiency bonus, so they've been consolidated into 4-level groups. Of course, the actual effects on the character are far-reaching. All their skills, saves, and attacks are worse. If they're an eldritch knight, they're losing out on a point of spell save DC as well.
Oh, it bears noting that since we're not doing take-backsies on ASIs, we're treating the fighter as having gained 11 hit points per level, whereas on the level where they bumped their constitution from 18 to 20 they might actually have gained as many as 27 and at 2nd level they could only have gained 5 if they'd had a constitution penalty.
3.5/Pathfinder Example: Simple Fighter
Level
Hit Points
BAB
Fortitude
Handle Animal
1
12/12
+1
+4
+4
2
7/7
+1
+1
+1
3
7/7
+1
+0
+1
4
1/7
+1
+1
+1
5
0/7
+1
+0
+1
6
0/7
+1
+1
+1
Total
27/47
4/6
6/7
7/9
This fighter has taken a bit of a tumble, and lost two levels. If we only use BAB, saves and skills, then he's going to be down 2 points of BAB (that's your extra attack down the drain), 1 point of fortitude bonus (for those counting at home, he's also losing 1 point of reflex and will) and 2 points of handle animal.
3.5/Pathfinder Example: Less Simple Fighter
Let's also take feats into account.
Level
Hit Points
BAB
1
1/12
Dodge, MobilityB
2
0/10
ToughnessB
3
0/7
Iron Will
4
0/7
Weapon Focus (Greatsword)B
5
0/7
-
6
0/7
Improved InitiativeB Weapon Specialisation (Greatsword)
Total
1/50
Dodge, Mobility, Toughness, Iron Will, Weapon Focus (Greatsword), Improved Initiative, Weapon Specialisation (Greatsword)
This is a fighter on his last hit point, and it shows. Apart from losing 5 points of BAB, 3 points of fortitude, 2 points of reflex, 4 points of will - he's lost the Iron Will feat as well as two points of will that he gets naturally - but also the attack and damage bonuses with the greatsword, and his initiative boost (I recommend not changing his position in the combat order to account for the change in initiative bonus, for the same reason that you don't retroactively chuck someone off a cliff if they get worse at climbing after they're already on a mountain, and for the same reason the spell nerveskitter works). Notably, when he loses Toughness as a feat, he doesn't lose 3 extra hit points (for the same reason that our grand, verbiose character we started off with wouldn't have lost any mana if he'd spent 3 or more points before losing a level).
What if we decide that only class features are lost (taking the radical view that bonus feats really are class features)? Well, in that case, he's only going to lose Improved Initiative, Weapon Focus and Toughness (and losing Toughness doesn't matter). However, he'll lose the prerequisite for one of his feats (namely, he'll lose Weapon Focus, the prerequisite for Weapon Specialisation), so he won't be able to use the feat (and claim the +2 damage bonus).
3.5/PF Example: Simple Sorcerer
Level
Hit Points
Spells Known
Caster Level
1
4/4
Acid Splash, Resistance, Mending, Detect Magic, Burning Hands, Magic Missile
1
2
1/2
Message
1
3
0/2
Alarm
1
4
0/2
Prestidigitation, Scorching Ray
1
Total
5/10
Acid Splash, Resistance, Mending, Detect Magic, Burning Hands, Magic Missile, Message, Alarm, Prestidigitation, Scorching Ray
2/4
Rather than trying to keep track of the sorcerer's spells/day, you can just let caster level reductions prevent them from casting their strongest spells, while eating away at their spells known. You could even just reduce their caster level. The other way of doing this, naturally, is more annoying:
3.5/PF Example: Moderately Complex Sorcerer
Level
Hit Points
Spells 1st
Spells 2nd
1
4/4
5/5
0
2
1/2
0/1
0
3
0/2
1/1
0
4
0/2
0/1
2/4
Total
5/10
3/6/8
0/0/4
So, let's have a look at what's happened. The sorcerer blasted away with two second-level spells and a first-level spell, which have all come from level 4. The sorcerer then takes 5 damage, losing two levels. She suddenly loses the 2/4 second-level spells from level 4, and the 0/1 first-level spells. She also loses access to the 1/1 first-level spell from 3rd level, so when she fires back with a first-level spell, she has to use the one from level 2.
This arguably actually gets easier if you use a wizard, because you can track which prepared spell the wizard's losing when:
3.5/PF Example: Fairly Simple Wizard
Level
Hit Points
Spells Prepared
1
3/4
Acid Splash, Resistance, Detect Magic, Burning Hands, Magic Missile
2
0/2
Mending, Alarm
Total
3/6
Acid Splash, Resistance, Detect Magic, Burning Hands, Magic Missile, (Mending), (Alarm)
The wizard's gone and cast Burning Hands, then taken 3 points of damage. Right now, this means they only have access to magic missile and some cantrips. When they get healed, they'll get back their mending and alarm spell (though if they'd cast the alarm, they wouldn't get that back, of course).
3.5's Spirit Shaman, and all of 5e's prepared casters, work slightly differently, though.
5e Example: Complex Wizard
Level
Hit Points
Proficiency Bonus
Spell Save DC
Class Features
Cantrips Known
Spells Prepared
Slots (1st)
Slots (2nd)
Slots (3rd)
1
6/6
+2
13
Spellcasting, Arcane Recovery
Acid Splash, Prestidigitation, True Strike
Burning Hands, Charm Person, Mage Armour, Magic Missile
2/2
0
0
2
4/4
0
0
Evocation Savant, Sculpt Spells
-
Alter Self
1/1
0
0
3
4/4
0
0
-
-
Scorching Ray
1/1
1/2
0
4
0/4
0
1
Ability Score Increase (Int, Int)
Mage Hand
Invisibility, Misty Step
0
1/1
0
5
0/4
+1
1
-
-
Fireball
0
0
2/2
Total
14/22
+2/+3
13/15
Spellcasting, Arcane Recovery, Evocation Savant, Sculpt Spells, Ability Score Increase (Int, Int)
Acid Splash, Prestidigitation, True Strike, Mage Hand
Burning Hands, Charm Person, Mage Armour, Magic Missile, Alter Self, Scorching Ray, Invisibility, Misty Step, Fireball
4/4/4
1/2/3
0/0/2
Naturally, the most complicated example by far is from 5e, a system designed for its simplicity. Ugh. Let's go through:
Hit points: 8 hit points are gone, and with them, 2 levels.
Proficiency bonus: Drops down to +2 with the loss of level 5.
Save DC: Drops by 2 - 1 for the loss of a point of proficiency, and 1 for the loss of the ASI. The wizard's proficient int-based skills also drop by 2.
Class Features: The ASI is gone, which wreaks havoc across the board. A DM would be perfectly within their rights to say that ASIs aren't lost, even if other class features are. Arcane Recovery is usable, but sees the wizard's level as 3, not 5. It'll only recover 2 levels of spells. Oh, and if you'd lost your evocation school features, no, you can't choose a new school when you get them back.
Cantrips: Mage hand is no longer a cantrip available to the wizard.
Spells Prepared: The main thing to note here is that yes, the wizard prepared a second-level spell at level 2. That's because there's no prohibition on a level 3 wizard prepping nothing but 2nd-level spells - essentially, there's no prohibition on them prepping spells like that under the base rules, so neither is there one under these rules. If they get knocked down to 2nd level exactly, they'll have a spell prepped that they can't cast.
Spell Slots: This works the way it did the last however-many times. This wizard got knocked down to 3rd level, before spending a 2nd-level spell slot. They couldn't use the one from level 4, because they'd lost level 4, so they used one of the ones from level 3.
And Another Thing...
There are some edge cases here. Let's have a look at them.
Various: I Have No Levels, and I Must Feature.
You can't lose anything from your first level, not even if you're on zero or negative hit points (you're staggered/disabled in 3.P, you have an ability such as Ferocity that lets you keep going on negatives, you have a class feature which matters while you're unconscious, etc).
Various: Wait, This Ability Does Nothing!
Some abilities, such as the Champion Fighter's Survivor ability (5e) and the Knight's Loyal Beyond Death ability (3.5) do nothing, while other abilities have a severely reduced effect, under this system if you're taking away everyone's class features. DMs should provide a case-by-case exemption for these features - they are never lost, no matter how low your hit points go. Further, in cases such as the Knight's Loyal Beyond Death (which uses Challenges, a limited resource like spell slots or our grand verbose friend's mana points), you should probably allow them to use resources from any level below the level at which you gain that feature, even if they'd normally be unavailable.
Alternatively, you could make those abilities do slightly differnet things. For example, Survivor could be changed to be unconditional (so, change it from "you regain hit points equal to 5 + your Constitution modifier if you have no more than half of your hit points left" to "you regain hit points equal to 5 + your Constitution modifier").
Various: Toughness, Improved Toughness and ASIs, oh my!
Pathfinder's Toughness, 3.5's Improved Toughness and permanent constitution increases all increase your hit points retroactively. To deal with this, if you are removing that type of ability as levels are lost (for example, if you're removing feats in the case of improved toughness), count the retroactive gain as gain for that level (so, if you gain +2 CON at 4th level, your hit points might go 8+5+5+9, instead of 8+5+5+5 if you chose +2 STR). If you are not removing that type of ability, count the retroactive gain, well, retroactively (so your hit points would go 9+6+6+6).
Various: This Seems Familiar...
Companions are weird.
Anything which gains levels, RHD, or whatever (animal companions, I'm looking at you) can tie their advancement and regression to their own HD. For example, in 3.5, a 7th-level druid's large viper companion has +2 hit dice. When those hit dice are both lost, so are the +1 to strength and dexterity, +2 to natural armour, the bonus trick and evasion ability that are tied to it (assuming you're taking away class features as levels are lost). Further, the viper's poison save DC, base attack bonus, saving throws and so forth are reduced (again, if you're doing that for characters in general).
Anything that doesn't gain levels or RHD conventionally has to use the hit points they gain when they gain class features as virtual hit dice. For example, in 3.5, a familiar has half its master's hit points, and as many HD as its master. Therefore, if the master gains 5, 3, 3 and 3 hit points, the familiar counts as having gained 2, 2, 1 and 2 (the rounding is weird because you have to round the total, not the number gained at the specific level). Rather than the master's effective level, the familiar's capabilities are tied to its virtual hit dice. Alternatively, you may decide that due to their magical or psionic nature (and the fact that keeping track of them is more trouble than it's worth), familiars and psicrystals are always at full strength.
3.5/PF/5e: Don't Lose Focus!
Can you concentrate on a spell you can no longer cast? What happens if your caster level changes during a spell's duration? I would go with "Yes" and "Nothing" respectively. This is consistent with the way that negative levels work in 3.P, the fact that a spell's duration can continue even after the death of the caster irrespective of edition, and the effects of the psionic power solicit psicrystal (the psicrystal can't manifest powers, but it can concentrate on them).
3.5/PF/5e: How Many Hit Dice?
These are sort of two different questions, because hit dice do something different in 5e from what they do in 3.P. In 5e, I would recommend not messing with a character's number of hit dice. If you really want to, though, you can use the same thing that we've been doing with spell slots with the current, current maximum and maximum.
In 3.5 and PF, I would recommend making characters lose hit dice. Not only does this give spells like cause fear a little longer in the sun, but it's a very easy way of making separate-level hit points meaningful without really adding any complexity.
3.5/PF/5e: Multiclass Problems
Apart from proficiencies (I really don't think that someone should lose their greatsword proficiency in the middle of a fight if you're trying to increase realism), you lose things from your multiclass levels the same way you would if you were single-classed.
3.5/PF/5e: Temporary Hit Points
I recommend treating temporary hit points as just a buffer, but you can also treat them as real hit points if you like.
3.5/PF/5e: The Problem with Constitution Damage (And Other Constitution Changes (or other ability score changes))
Constitution damage is a pain under this system. There's two ways to handle it: either you can calculate it properly, or you can just recalculate the current hit points, and not the hit point thresholds. The same is true of other constitution changes, but if they're the kind of change that tends to last a long time (say, the kind that requires attunement), it's best to recalculate properly.
The effects of a barbarian's rage in 3.P (or indeed in 5e, but that's less relevant to this case) is determined when you enter the rage, so you can't bootstrap your own rage (the +4 constitution takes you up to 11th level, and then that gives you another +2, which takes you up to 20th level, which gives you another +2... no, no, you can't do that).
The same general stuff applies to things like intelligence modifiers and bonus spell slots/power points/spells prepared. If something is level+ability modifier or similar, take them off the level 1 list if the ability modifier is reduced.
3.5/PF: Negative Levels?
You may wish to change "Each negative level gives a creature the following penalties: -1 penalty on attack rolls, saving throws, skill checks, ability checks; loss of 5 hit points; and -1 to effective level (for determining the power, duration, DC, and other details of spells or special abilities). In addition, a spellcaster loses one spell or spell slot from the highest spell level castable." to "Each negative level removes the highest level a character has attained, including the current and maximum hit points from that level. Negative levels are considered first, then hit point loss." Consider, for example, this fighter:
Level
Hit Points
BAB
1
10(-4)/10
+1
2
5(-5)/6
+1
3
0/9
+1
Total
15(-9)/25 16
+1
When the fighter takes a negative level, it's functionally identical to losing nine hit points, on top of the ten hit points the fighter has already lost. This actually means that the fighter loses two levels (and I bet that taking toughness on your highest level seemed like a good idea at the time!). Notice that even though the fighter had already lost his highest level, the negative level still steals the highest level, then the hit point damage is applied afterwards.
3.5/PF: Nonlethal Damage
Count nonlethal damage as though it were lethal when using these rules. Say you have 29/45 hit points and have taken 14 points of nonlethal damage. When determining your effective level, calculate it as though you had 15 hit points remaining.
5e: Behold, the Power of Rituals!
No, you can't cast a spell as a ritual from your spellbook or your prepared spells if it's too high a level to cast it normally. For example, if you're 11th-level, have instant summons in your book, and get knocked back down to 1st level, you can't cast it as a ritual. There's nothing stopping your druid prepping commune with nature as one of the spells tied to their first druid level, but that doesn't mean they can cast it as a ritual if they get knocked back to basic.
Stars Without Number: No Hit Points to Declare
Because of the weird (and arguably stupid) way that SWN handles hit points, it's possible not to gain any hit points at a given level. I recommend that, if you're really hellbent on rolling for hit points, you roll the new hit die and add it, rather than re-rolling the entire total and only increasing it if you roll higher than you had last level.
So, in 3.5, Pathfinder and 5th edition, there's no difference between being at 1 hit point and being at full hit points, except for how much more damage you have to take to knock you to 0 (or -1 in 3.P, -10 in 3.5, or -CON in PF). The exceptions, mainly Power Words, are not particularly common or commonly-used (notwithstanding the monstrously-overpowered PW: Pain, which deals... hit point damage). This may not be a problem if you're not angling for a particularly realistic game, but it can shake suspension of disbelief when your worn-down fighter who'll be knocked out by the next punch that lands is leaping, bounding and swinging away the same way he was before he took seventeen greatswords to the face (replace with "mitigated the damage from seventeen greatswords by spending hit points" or "spent hit points to avoid getting hit by seventeen greatswords" to taste, depending on how you see hit points).
I'm sure you can use these house rules for any edition, or even any game which has levels and in which you gain hit points as you level up.
The Rule in Brief
The hit points you gain at a particular level are specific to that level. When you run out of hit points for a level, you lose some of the benefits of that level. That's it! You, the DM using this rule, can decide with your group how much goes away when you lose a level (it gets a little messy with spell slots), but the important part is that you shouldn't make people lose hit points because they lost hit points.
Non-Specific Example
Let's consider a class table, from a simple system I've made up for this example:
Level
Hit Points
Verbosity
Grandeur
Mana Per Day
1
6+healthiness
2+truthiness
3
4
2
9+2*healthiness
3+truthiness
5
7
3
12+3*healthiness
4+truthiness
7
10
Suppose a character is third level, and has a healthiness of 2 and a truthiness of 3. They don't just mark their hit points as 18, their verbosity as 7, their grandeur also as 7, and their mana per day as 10. Instead, they write it out in a table, similar to the class table above (the italics here represent pencil):
Level
Hit Points
Verbosity
Grandeur
Mana
1
8/8
5
3
4/4
2
5/5
1
2
3/3
3
5/5
1
2
3/3
Total
18/18
7/7
7/7
10/10/10
The table now shows the increases gained at each level. It also shows the total of all the values. Because Verbosity and Grandeur aren't a resource you can spend (they're like THAC0, BAB, Proficiency bonus, or your preferred way of determining how good you are at hitting things) they'll only go down when you lose levels. Mana points can be lost either from using them up, or losing them when you lose levels. So, there are three values: the current value ("How much mana do I have right now?", the current maximum "What's my maximum mana value at the current level, given my hit point total"), and the actual maximum ("What would be the maximum amount of mana I could have, at my full level, if my hit point total were full?"). For hit points, you don't bother with the current maximum because we assume you can be healed back up a level, so the current maximum isn't relevant. Now, let's start off by supposing the character loses 7 hit points, and therefore loses a level:
Level
Hit Points
Verbosity
Grandeur
Mana
1
8/8
5
3
4/4
2
3/5
1
2
3/3
3
0/5
1
2
3/3
Total
11/18
6/7
5/7
7/7/10
Let's look at the total line, and the verbosity stat to start off. Three points of verbosity come from your truthiness score, two of them from first level, and one from second level. That makes six at second level. Sure enough, you're down to second level, and down to six verbosity. Another way of looking at it is that you had seven verbosity, one of which came from level 3. You've lost level 3, so now you're down to six verbosity. You lose 2 points of grandeur the same way. Your current and maximum mana similarly flop 3 points apiece. The hit points have gone down by 7 - nothing unusual there.
Now, let's look at the hit points on each level. Your level 3 hit points are all gone, and your level 2 hit points are down by 2. So far, so good. Let's cast a 2 mana spell in revenge, smiting our foe:
Level
Hit Points
Verbosity
Grandeur
Mana
1
8/8
5
3
4/4
2
3/5
1
2
1/3
3
0/5
1
2
3/3
Total
11/18
6/7
5/7
5/7/10
Previously, we had 7/7/10 mana. Now, it's 5/7/10. If you look at the 2nd-level and 3rd-level rows, you'll see that we can't access the 3 mana we gain at 3rd level, because we're not at 3rd level. We use the 2nd-level mana instead. However, if we'd got the first hit in, the table would look like this:
Level
Hit Points
Verbosity
Grandeur
Mana
1
8/8
5
3
4/4
2
3/5
1
2
3/3
3
0/5
1
2
1/3
Total
11/18
6/7
5/7
7/7/10
We lost 2 mana from the third level when we cast the spell. We then lost all our current mana and current maximum mana from level 3. That means that we lose 1 current, and 3 current maximum mana. They both, therefore, drop to 7.
5e Example: Super-Simple Fighter
Level
Hit Points
Proficiency
1-4
48/48
+2
5-8
44/44
+1
9-12
44/44
+1
13-16
32/44
+1
17
0/11
+1
Total
168/191
+5/+6
This is a relatively easy start. The 17th-level fighter here has actually lost two levels, but from this table, you can't tell because it doesn't matter. All that matters is how many hit points apply to each point of proficiency bonus, so they've been consolidated into 4-level groups. Of course, the actual effects on the character are far-reaching. All their skills, saves, and attacks are worse. If they're an eldritch knight, they're losing out on a point of spell save DC as well.
Oh, it bears noting that since we're not doing take-backsies on ASIs, we're treating the fighter as having gained 11 hit points per level, whereas on the level where they bumped their constitution from 18 to 20 they might actually have gained as many as 27 and at 2nd level they could only have gained 5 if they'd had a constitution penalty.
3.5/Pathfinder Example: Simple Fighter
Level
Hit Points
BAB
Fortitude
Handle Animal
1
12/12
+1
+4
+4
2
7/7
+1
+1
+1
3
7/7
+1
+0
+1
4
1/7
+1
+1
+1
5
0/7
+1
+0
+1
6
0/7
+1
+1
+1
Total
27/47
4/6
6/7
7/9
This fighter has taken a bit of a tumble, and lost two levels. If we only use BAB, saves and skills, then he's going to be down 2 points of BAB (that's your extra attack down the drain), 1 point of fortitude bonus (for those counting at home, he's also losing 1 point of reflex and will) and 2 points of handle animal.
3.5/Pathfinder Example: Less Simple Fighter
Let's also take feats into account.
Level
Hit Points
BAB
1
1/12
Dodge, MobilityB
2
0/10
ToughnessB
3
0/7
Iron Will
4
0/7
Weapon Focus (Greatsword)B
5
0/7
-
6
0/7
Improved InitiativeB Weapon Specialisation (Greatsword)
Total
1/50
Dodge, Mobility, Toughness, Iron Will, Weapon Focus (Greatsword), Improved Initiative, Weapon Specialisation (Greatsword)
This is a fighter on his last hit point, and it shows. Apart from losing 5 points of BAB, 3 points of fortitude, 2 points of reflex, 4 points of will - he's lost the Iron Will feat as well as two points of will that he gets naturally - but also the attack and damage bonuses with the greatsword, and his initiative boost (I recommend not changing his position in the combat order to account for the change in initiative bonus, for the same reason that you don't retroactively chuck someone off a cliff if they get worse at climbing after they're already on a mountain, and for the same reason the spell nerveskitter works). Notably, when he loses Toughness as a feat, he doesn't lose 3 extra hit points (for the same reason that our grand, verbiose character we started off with wouldn't have lost any mana if he'd spent 3 or more points before losing a level).
What if we decide that only class features are lost (taking the radical view that bonus feats really are class features)? Well, in that case, he's only going to lose Improved Initiative, Weapon Focus and Toughness (and losing Toughness doesn't matter). However, he'll lose the prerequisite for one of his feats (namely, he'll lose Weapon Focus, the prerequisite for Weapon Specialisation), so he won't be able to use the feat (and claim the +2 damage bonus).
3.5/PF Example: Simple Sorcerer
Level
Hit Points
Spells Known
Caster Level
1
4/4
Acid Splash, Resistance, Mending, Detect Magic, Burning Hands, Magic Missile
1
2
1/2
Message
1
3
0/2
Alarm
1
4
0/2
Prestidigitation, Scorching Ray
1
Total
5/10
Acid Splash, Resistance, Mending, Detect Magic, Burning Hands, Magic Missile, Message, Alarm, Prestidigitation, Scorching Ray
2/4
Rather than trying to keep track of the sorcerer's spells/day, you can just let caster level reductions prevent them from casting their strongest spells, while eating away at their spells known. You could even just reduce their caster level. The other way of doing this, naturally, is more annoying:
3.5/PF Example: Moderately Complex Sorcerer
Level
Hit Points
Spells 1st
Spells 2nd
1
4/4
5/5
0
2
1/2
0/1
0
3
0/2
1/1
0
4
0/2
0/1
2/4
Total
5/10
3/6/8
0/0/4
So, let's have a look at what's happened. The sorcerer blasted away with two second-level spells and a first-level spell, which have all come from level 4. The sorcerer then takes 5 damage, losing two levels. She suddenly loses the 2/4 second-level spells from level 4, and the 0/1 first-level spells. She also loses access to the 1/1 first-level spell from 3rd level, so when she fires back with a first-level spell, she has to use the one from level 2.
This arguably actually gets easier if you use a wizard, because you can track which prepared spell the wizard's losing when:
3.5/PF Example: Fairly Simple Wizard
Level
Hit Points
Spells Prepared
1
3/4
Acid Splash, Resistance, Detect Magic, Burning Hands, Magic Missile
2
0/2
Mending, Alarm
Total
3/6
Acid Splash, Resistance, Detect Magic, Burning Hands, Magic Missile, (Mending), (Alarm)
The wizard's gone and cast Burning Hands, then taken 3 points of damage. Right now, this means they only have access to magic missile and some cantrips. When they get healed, they'll get back their mending and alarm spell (though if they'd cast the alarm, they wouldn't get that back, of course).
3.5's Spirit Shaman, and all of 5e's prepared casters, work slightly differently, though.
5e Example: Complex Wizard
Level
Hit Points
Proficiency Bonus
Spell Save DC
Class Features
Cantrips Known
Spells Prepared
Slots (1st)
Slots (2nd)
Slots (3rd)
1
6/6
+2
13
Spellcasting, Arcane Recovery
Acid Splash, Prestidigitation, True Strike
Burning Hands, Charm Person, Mage Armour, Magic Missile
2/2
0
0
2
4/4
0
0
Evocation Savant, Sculpt Spells
-
Alter Self
1/1
0
0
3
4/4
0
0
-
-
Scorching Ray
1/1
1/2
0
4
0/4
0
1
Ability Score Increase (Int, Int)
Mage Hand
Invisibility, Misty Step
0
1/1
0
5
0/4
+1
1
-
-
Fireball
0
0
2/2
Total
14/22
+2/+3
13/15
Spellcasting, Arcane Recovery, Evocation Savant, Sculpt Spells, Ability Score Increase (Int, Int)
Acid Splash, Prestidigitation, True Strike, Mage Hand
Burning Hands, Charm Person, Mage Armour, Magic Missile, Alter Self, Scorching Ray, Invisibility, Misty Step, Fireball
4/4/4
1/2/3
0/0/2
Naturally, the most complicated example by far is from 5e, a system designed for its simplicity. Ugh. Let's go through:
Hit points: 8 hit points are gone, and with them, 2 levels.
Proficiency bonus: Drops down to +2 with the loss of level 5.
Save DC: Drops by 2 - 1 for the loss of a point of proficiency, and 1 for the loss of the ASI. The wizard's proficient int-based skills also drop by 2.
Class Features: The ASI is gone, which wreaks havoc across the board. A DM would be perfectly within their rights to say that ASIs aren't lost, even if other class features are. Arcane Recovery is usable, but sees the wizard's level as 3, not 5. It'll only recover 2 levels of spells. Oh, and if you'd lost your evocation school features, no, you can't choose a new school when you get them back.
Cantrips: Mage hand is no longer a cantrip available to the wizard.
Spells Prepared: The main thing to note here is that yes, the wizard prepared a second-level spell at level 2. That's because there's no prohibition on a level 3 wizard prepping nothing but 2nd-level spells - essentially, there's no prohibition on them prepping spells like that under the base rules, so neither is there one under these rules. If they get knocked down to 2nd level exactly, they'll have a spell prepped that they can't cast.
Spell Slots: This works the way it did the last however-many times. This wizard got knocked down to 3rd level, before spending a 2nd-level spell slot. They couldn't use the one from level 4, because they'd lost level 4, so they used one of the ones from level 3.
And Another Thing...
There are some edge cases here. Let's have a look at them.
Various: I Have No Levels, and I Must Feature.
You can't lose anything from your first level, not even if you're on zero or negative hit points (you're staggered/disabled in 3.P, you have an ability such as Ferocity that lets you keep going on negatives, you have a class feature which matters while you're unconscious, etc).
Various: Wait, This Ability Does Nothing!
Some abilities, such as the Champion Fighter's Survivor ability (5e) and the Knight's Loyal Beyond Death ability (3.5) do nothing, while other abilities have a severely reduced effect, under this system if you're taking away everyone's class features. DMs should provide a case-by-case exemption for these features - they are never lost, no matter how low your hit points go. Further, in cases such as the Knight's Loyal Beyond Death (which uses Challenges, a limited resource like spell slots or our grand verbose friend's mana points), you should probably allow them to use resources from any level below the level at which you gain that feature, even if they'd normally be unavailable.
Alternatively, you could make those abilities do slightly differnet things. For example, Survivor could be changed to be unconditional (so, change it from "you regain hit points equal to 5 + your Constitution modifier if you have no more than half of your hit points left" to "you regain hit points equal to 5 + your Constitution modifier").
Various: Toughness, Improved Toughness and ASIs, oh my!
Pathfinder's Toughness, 3.5's Improved Toughness and permanent constitution increases all increase your hit points retroactively. To deal with this, if you are removing that type of ability as levels are lost (for example, if you're removing feats in the case of improved toughness), count the retroactive gain as gain for that level (so, if you gain +2 CON at 4th level, your hit points might go 8+5+5+9, instead of 8+5+5+5 if you chose +2 STR). If you are not removing that type of ability, count the retroactive gain, well, retroactively (so your hit points would go 9+6+6+6).
Various: This Seems Familiar...
Companions are weird.
Anything which gains levels, RHD, or whatever (animal companions, I'm looking at you) can tie their advancement and regression to their own HD. For example, in 3.5, a 7th-level druid's large viper companion has +2 hit dice. When those hit dice are both lost, so are the +1 to strength and dexterity, +2 to natural armour, the bonus trick and evasion ability that are tied to it (assuming you're taking away class features as levels are lost). Further, the viper's poison save DC, base attack bonus, saving throws and so forth are reduced (again, if you're doing that for characters in general).
Anything that doesn't gain levels or RHD conventionally has to use the hit points they gain when they gain class features as virtual hit dice. For example, in 3.5, a familiar has half its master's hit points, and as many HD as its master. Therefore, if the master gains 5, 3, 3 and 3 hit points, the familiar counts as having gained 2, 2, 1 and 2 (the rounding is weird because you have to round the total, not the number gained at the specific level). Rather than the master's effective level, the familiar's capabilities are tied to its virtual hit dice. Alternatively, you may decide that due to their magical or psionic nature (and the fact that keeping track of them is more trouble than it's worth), familiars and psicrystals are always at full strength.
3.5/PF/5e: Don't Lose Focus!
Can you concentrate on a spell you can no longer cast? What happens if your caster level changes during a spell's duration? I would go with "Yes" and "Nothing" respectively. This is consistent with the way that negative levels work in 3.P, the fact that a spell's duration can continue even after the death of the caster irrespective of edition, and the effects of the psionic power solicit psicrystal (the psicrystal can't manifest powers, but it can concentrate on them).
3.5/PF/5e: How Many Hit Dice?
These are sort of two different questions, because hit dice do something different in 5e from what they do in 3.P. In 5e, I would recommend not messing with a character's number of hit dice. If you really want to, though, you can use the same thing that we've been doing with spell slots with the current, current maximum and maximum.
In 3.5 and PF, I would recommend making characters lose hit dice. Not only does this give spells like cause fear a little longer in the sun, but it's a very easy way of making separate-level hit points meaningful without really adding any complexity.
3.5/PF/5e: Multiclass Problems
Apart from proficiencies (I really don't think that someone should lose their greatsword proficiency in the middle of a fight if you're trying to increase realism), you lose things from your multiclass levels the same way you would if you were single-classed.
3.5/PF/5e: Temporary Hit Points
I recommend treating temporary hit points as just a buffer, but you can also treat them as real hit points if you like.
3.5/PF/5e: The Problem with Constitution Damage (And Other Constitution Changes (or other ability score changes))
Constitution damage is a pain under this system. There's two ways to handle it: either you can calculate it properly, or you can just recalculate the current hit points, and not the hit point thresholds. The same is true of other constitution changes, but if they're the kind of change that tends to last a long time (say, the kind that requires attunement), it's best to recalculate properly.
The effects of a barbarian's rage in 3.P (or indeed in 5e, but that's less relevant to this case) is determined when you enter the rage, so you can't bootstrap your own rage (the +4 constitution takes you up to 11th level, and then that gives you another +2, which takes you up to 20th level, which gives you another +2... no, no, you can't do that).
The same general stuff applies to things like intelligence modifiers and bonus spell slots/power points/spells prepared. If something is level+ability modifier or similar, take them off the level 1 list if the ability modifier is reduced.
3.5/PF: Negative Levels?
You may wish to change "Each negative level gives a creature the following penalties: -1 penalty on attack rolls, saving throws, skill checks, ability checks; loss of 5 hit points; and -1 to effective level (for determining the power, duration, DC, and other details of spells or special abilities). In addition, a spellcaster loses one spell or spell slot from the highest spell level castable." to "Each negative level removes the highest level a character has attained, including the current and maximum hit points from that level. Negative levels are considered first, then hit point loss." Consider, for example, this fighter:
Level
Hit Points
BAB
1
10(-4)/10
+1
2
5(-5)/6
+1
3
0/9
+1
Total
15(-9)/25 16
+1
When the fighter takes a negative level, it's functionally identical to losing nine hit points, on top of the ten hit points the fighter has already lost. This actually means that the fighter loses two levels (and I bet that taking toughness on your highest level seemed like a good idea at the time!). Notice that even though the fighter had already lost his highest level, the negative level still steals the highest level, then the hit point damage is applied afterwards.
3.5/PF: Nonlethal Damage
Count nonlethal damage as though it were lethal when using these rules. Say you have 29/45 hit points and have taken 14 points of nonlethal damage. When determining your effective level, calculate it as though you had 15 hit points remaining.
5e: Behold, the Power of Rituals!
No, you can't cast a spell as a ritual from your spellbook or your prepared spells if it's too high a level to cast it normally. For example, if you're 11th-level, have instant summons in your book, and get knocked back down to 1st level, you can't cast it as a ritual. There's nothing stopping your druid prepping commune with nature as one of the spells tied to their first druid level, but that doesn't mean they can cast it as a ritual if they get knocked back to basic.
Stars Without Number: No Hit Points to Declare
Because of the weird (and arguably stupid) way that SWN handles hit points, it's possible not to gain any hit points at a given level. I recommend that, if you're really hellbent on rolling for hit points, you roll the new hit die and add it, rather than re-rolling the entire total and only increasing it if you roll higher than you had last level.