"I am still here."

[Xaroth]

In today's post, I'd like to talk about the thought of bosses that have "phases". I remember reading something a long while back, an inspired post where someone talked about the idea of slaying a boss, the party would be excited and checking for loot and asking for experience, but then all they would get in response is the boss arising from the floor, stating "I am still here", at which point they have even more abilities at their disposal.

I was wondering, if anybody has done something like this using 3.5e books, how did you go about this? What was the encounter like, and how many phases did the boss have? Aside from homebrewing it up or using some kinda modification of Death Pact, I'm not sure how I'd do it below 16th level.

[Krazzman]

I did something similar in my campaign a while back. It worked rather well and the party was quite surprised.

Encounter was some sort of remorhaz or how they are spelled that was stricken by The Darkness (a corrupting affliction) that spawned 2 shadowy humanoid slobs of stuff after killing the remorhaz.

Planned my end boss to have 3 such phases.
Corrupted Orc Warpriest//Bloodrager upon death he comes back as an advanced shadowy corrupted orc and after that he spawns The Darkness herself.

Never came to that encounter due to writers block and such stuff. The first encounter though ran really well.

[Buufreak]

I'm not going to lie, based on the title I really hoped this was going to be about FNAF convertions.

More to the point, I haven't found a good way in 3.5 that I actually like using, but I'm certain there are a list of spells that could be used for this. Contingency comes to mind, but I also remember something from (I think) Elder Evils. The general effect was a dark transformation at the cost of dying soonish.

[Xaroth]

I did something similar in my campaign a while back. It worked rather well and the party was quite surprised.

Encounter was some sort of remorhaz or how they are spelled that was stricken by The Darkness (a corrupting affliction) that spawned 2 shadowy humanoid slobs of stuff after killing the remorhaz.

Planned my end boss to have 3 such phases.
Corrupted Orc Warpriest//Bloodrager upon death he comes back as an advanced shadowy corrupted orc and after that he spawns The Darkness herself.

Never came to that encounter due to writers block and such stuff. The first encounter though ran really well.

Damn, writer's block sucks. Something about D&D prevents me from having WB with it, and goddamn am I thankful for that.


I'm not going to lie, based on the title I really hoped this was going to be about FNAF convertions.

More to the point, I haven't found a good way in 3.5 that I actually like using, but I'm certain there are a list of spells that could be used for this. Contingency comes to mind, but I also remember something from (I think) Elder Evils. The general effect was a dark transformation at the cost of dying soonish.

Honestly, the FNAF guys would just be possessed constructs, but regardless, that's not the point of the thread. I could probably throw them into my VG characters thread, though.

[johnbragg]

You may want to take a look athttp://theangrygm.com/return-of-the-son-of-the-dd-boss-fight-now-in-5e/, also known as "The orc is two orcs."

Mechanically, two separate, identical creatures occupying the same space. The second creature cannot be attacked until the first one is out of hit points. Or, looked at from the other end of the telescope, a single creature with two pools of hit points that gets double actions until one hit point pool is depleted. That's "The Orc Is Two ORcs" vanilla version.

Building on that idea, you could make the two creatures non-identical. Take a basic brute like an ogre or hill giant, and grab two different sets of templates. In Phase I, Half-Fiendish giving flight and a claw/claw/bite attack routine. Maybe with doubled-actions, maybe not.

Once the party has run through the first pool of hit points, the creature crashes to earth, (wings are too damaged to fly), and goes through a magical metamorphosis. In Phase II, some other template boosting melee combat, or just some class levels and magic gear.

EDIT: It helps a lot, for me at least, if A) the change in capabilities is somewhat logical (wound a flyer badly enough and it's grounded, bring down the pyromancer thematic caster, and it rises as a fire elemental, etc. Crack open the dog-sized spider's carapace, and a swarm of a bazillion spiders spills out.) and B) the second-stage fight uses different capabilities than the first-stage fight (flying archer --> ground-based melee weapon, Medium-sized spider --> spider swarm, etc)

You want that balance of "Yay! we made progress" and "Oh dear, what now?"

[Doctor Despair]

Well, you could do it this way: have the big bad either have a contingent spell of reanimate that rezz's them in a safe location on death or have the big bad be a lich whose phylactery is in a demiplane of fast-time, depending on how OP the party is. Add more contingent spells to power him up as needed. For example, the party is searching for loot and he dimensional doors/greater teleports behind them. He says "I am still here" before casting some empowered fear effect in a surprise round.

[Calthropstu]

Spontaneous undead creation would do the trick. Demonic possession would also work for this.

[asuev]

Craft contingent spell from the DMG2 (?) is probably the ideal way to do this. All it costs at that point is resources.

A craft contingent True Res is pretty nasty and has been used by my DM on occasion for our BBEG's.

[Calthropstu]

There is also simulacrum.

"The person you were fighting turns to dust upon defeat. A man identical to him in every respect steps out of a nearby door and says 'Did you really think it would be that easy?' Combat begins anew, roll initiative."

The entire time you were fighting his simulacrum, he was casting buffs and summon spells. Prepare for the worst.

[Crake]

There is also simulacrum.

"The person you were fighting turns to dust upon defeat. A man identical to him in every respect steps out of a nearby door and says 'Did you really think it would be that easy?' Combat begins anew, roll initiative."

The entire time you were fighting his simulacrum, he was casting buffs and summon spells. Prepare for the worst.

Problem with a simulacrum though is it's half the power of the person it's duplicating, so either the first fight would have been a hilarious pushover, or the second fight is going to be a tpk.

[johnbragg]

Problem with a simulacrum though is it's half the power of the person it's duplicating, so either the first fight would have been a hilarious pushover, or the second fight is going to be a tpk.

OR, y'know, both.

[Doctor Despair]

Problem with a simulacrum though is it's half the power of the person it's duplicating, so either the first fight would have been a hilarious pushover, or the second fight is going to be a tpk.

Ice Assassin, then? And the reason he doesn't have more than one is that he has a healthy respect for his own craftiness and thinks his clones would outsmart him if there were more than one?

[johnbragg]

Ice Assassin, then? And the reason he doesn't have more than one is that he has a healthy respect for his own craftiness and thinks his clones would outsmart him if there were more than one?

Big-picture question: Do you WANT the Phase II opponent to be a carbon-copy of the Phase I opponent, just facing the party at full health & fully buffed while the party is worn down?

[Xaroth]

Big-picture question: Do you WANT the Phase II opponent to be a carbon-copy of the Phase I opponent, just facing the party at full health & fully buffed while the party is worn down?

Not sure about Doc Despy but I was actually thinking a plethora of different abilities, maybe even a different fighting style entirely. Though I like the "The Orc is Two Orcs" thing, just from the sound of it.

I'll take a look into craft contingent spell. Might be what I'm looking for, but I was kinda wanting to avoid true rez?

[warmachine]

Problem with a simulacrum though is it's half the power of the person it's duplicating, so either the first fight would have been a hilarious pushover, or the second fight is going to be a tpk.
I say the former is a good idea. At the level bosses can cast Simulacrum, players will know of the spell, or at least magic can break expectations. The tactical players should realise the real target is not present and it's up to them to work what to do about it. When the real target appears, it makes a good story about how the smart characters were ready.

Having the boss say something like "You killed my simulacrum. He was my best friend and you shall pay for that!", could explain why the boss isn't running away.

[johnbragg]

Not sure about Doc Despy but I was actually thinking a plethora of different abilities, maybe even a different fighting style entirely. Though I like the "The Orc is Two Orcs" thing, just from the sound of it.

I'll take a look into craft contingent spell. Might be what I'm looking for, but I was kinda wanting to avoid true rez?

OK. Also, decide how important it is to you and your group that the enemies stick to the rules. Is it okay if they can be built in ways that PCs just can't be built? Either because DM fiat, or by building a balancing/resource mechanism that means "Yes, you COULD build your character to do that, (ie PCs and NPCs obey the same rules of quasi-physics) but it would basically mean giving up being an adventurer and spending 12 hours a day meditating at a shrine in your lair to generate the psychic energy to power the effects."

[Crake]

Not sure about Doc Despy but I was actually thinking a plethora of different abilities, maybe even a different fighting style entirely. Though I like the "The Orc is Two Orcs" thing, just from the sound of it.

I'll take a look into craft contingent spell. Might be what I'm looking for, but I was kinda wanting to avoid true rez?

When it comes to contingent spells and bringing people back from the dead, I actually prefer a contingent revivify and heal spell, bringing the character back up to at least 109hp, or up to 149 if the CL is 15, prevents level loss, and heals any ability score damage and whatnot as well, almost certainly putting the boss back to full, or close to full effectiveness.

As you said though, that's not quite as actually mixing up the abilities afterward.

Cal mentioned possession, and I second that one, it makes for a really varied fight, going from one set of abilities of the possessed being, to a completely different set for the demon itself. It also lets you have a decent spike in difficulty by having the demon possess a lower CR creature, being defeated in that form, then releasing the possession and fighting with his demonic powers.

[Flickerdart]

The second stage of the boss tends to be weaker than the first. After all, the PCs just blew all their cool moves!

Contingent animate dead is a good one, or you can have the BBEG come back as a ghost.

[Vogie]

There is also simulacrum.

"The person you were fighting turns to dust upon defeat. A man identical to him in every respect steps out of a nearby door and says 'Did you really think it would be that easy?' Combat begins anew, roll initiative."

The entire time you were fighting his simulacrum, he was casting buffs and summon spells. Prepare for the worst.

That reminds me of that Simulacrum Cheese thread (http://www.giantitp.com/forums/showthread.php?261434-Cheese-with-Simulacrum)from a while back...

[Calthropstu]

That reminds me of that Simulacrum Cheese thread (http://www.giantitp.com/forums/showthread.php?261434-Cheese-with-Simulacrum)from a while back...

Having a single simulacrum of yourself to engage enemies while you buff up (or escape) is not, in any definition, cheese. It's actually the intended usage I would believe.

[ComaVision]

I did the final fight in Red Hand of Doom as a two phase fight. If you're unfamiliar, the last boss of the module is a Half-Dragon Hobgoblin Sorcerer and when he dies an Aspect of Tiamat is supposed to come through a portal and continue the fight. Instead, I had the Sorcerer dying finish the ritual and he became the vessel for the Aspect. The party took it pretty well, despite all but one character dying.

I don't know any way I could have done that RAW though, sorry.

[flappeercraft]

A good one could be to use an Ice assassin with same wealth equivalent magic item total but different for the Ice assassin and the BBEG. If the BBEG is a prepared caster then mix up the prepared spells and you can always DCFS the BBEG for both the Ice assassinand BBEG to have different feats.

[Zancloufer]

Surprised this one has come up yet:

First fight is what seems to be a fairly intelligent Huge+ metal golem. Maybe with some siege weapons attached to it. After "defeating" it (would pretty much be a golem with more firepower and less stupid) it turns out the golem was piloted. Cue a person bursting out of it and begin phase two of the battle.

Heck make the pilot a Warblade or something that abuses manoeuvres while the golem was a giant pile of HP and siege weapons. Two very different types of battle that flow together well from a story perspective.

[Vogie]

Having a single simulacrum of yourself to engage enemies while you buff up (or escape) is not, in any definition, cheese. It's actually the intended usage I would believe.

That is correct. That is why the thread I linked was NOT involving simulacrums of oneself.

[Bucky]

For lower level campaigns you can use the Feign Death ability exactly as intended, provided the boss has enough HP.

[The Insanity]

Had something like that. The boss was a samurai. She fought the PCs one-handed while holding her katana's scabard in the other. When they dropped her below half health, she proclaimed "you're worthy opponents", dropped her scabard, started two-handing the katana and use her highest level maneuvers.

[martixy]

Somewhat offtopic, since I don't have much to contribute, have any of you heard of, seen or played Furi?

Boss battles are always tense and fun. Multi-stage boss battles multiply so.

Another option is objectives that must be completed to make the boss vulnerable/weak enough to be taken down. Works especially well with larger ones.
Haven't played Shadow of the Colossus, but didn't they do something like that?

[Mordaedil]

Surprised this one has come up yet:

First fight is what seems to be a fairly intelligent Huge+ metal golem. Maybe with some siege weapons attached to it. After "defeating" it (would pretty much be a golem with more firepower and less stupid) it turns out the golem was piloted. Cue a person bursting out of it and begin phase two of the battle.

Heck make the pilot a Warblade or something that abuses manoeuvres while the golem was a giant pile of HP and siege weapons. Two very different types of battle that flow together well from a story perspective.

That's so ****ing cool. I'm gonna point my DM at this.

[John Longarrow]

Best one of these I've seen...

Distinctive Boss in distinctive armor with distinct voice and speech mannerisms.
Party eventually drops boss. Boss lays there and then shifts back to look like some average Joe.

Party faces Boss again, in distinctive armor with distinct voice/look/speech.
Party beats boss. Boss shifts to look like a different average Joe.

Behind the scenes. BBEG has multiple "Alter self" items that are customized to specific subordinates. Each wears one to appear as "The boss". None are as powerful as he is and each fights a bit differently.

Turned out to be really annoying after the third time though.

[Korahir]

Didn't look in any detail but wouldn't it be possible for a creature with the swallow whole ability to swallow the boss, then fight the group and the real boss breaks free only after defeat of the creature who swallowed the boss. Of course the boss must be immune to the damage done by the swallow whole ability. I know it is a stupid idea but hey, it is creative.

[Xaroth]

Didn't look in any detail but wouldn't it be possible for a creature with the swallow whole ability to swallow the boss, then fight the group and the real boss breaks free only after defeat of the creature who swallowed the boss. Of course the boss must be immune to the damage done by the swallow whole ability. I know it is a stupid idea but hey, it is creative.

So...acid immunity and damage reduction, along with fast healing to make up for whatever damage remains?

I find it funny imagining a T-Rex storming about the battlefield, with this guy in his stomach wielding a weapon and so on so forth, and he keeps checking his pocket watch like "Is it my turn yet?"

T-Rex's stomach in response: *GURGLE*

[Korahir]

Had a brief look into pathfinder monsters: there are a lot of swallow whole monsters out there. The most surprising would be the Huge Toad who is large and CR 2. Does 1d4 bludgeoning damage when you get swallowed so every DR 5 and every Regeneration 5 works.

[Mr Adventurer]

I once ran an encounter where the party fought what they thought was Elminster, but turned out to be two halflings, one on the other's shoulders.

Through use of a custom spell (Appear as Elminster), the half-fiend halfling wizard used his half-dragon barbarian brother's movement speed. When the wizard died, the barbarian burst out of Elminsters robes in a rage.

[Mr Adventurer]

Oh, once, also, we fought the Tarrasque, who had the Half-fiendish template, and was wearing a Headband of Magnificence +6. When we killed it, it rose as a zombie (with 96 HD!).

[Xaroth]

Oh, once, also, we fought the Tarrasque, who had the Half-fiendish template, and was wearing a Headband of Magnificence +6. When we killed it, it rose as a zombie (with 96 HD!).

Why, though?

[Morphic tide]

One word: Druids.

Druids have quite a few mechanics supporting the idea of a multi-stage bossfight. I'm not sure if such a thing exists in 3.5, but there are Druid variants that restore their HP on a Wildshape. Have them not use Natural Spell and you can force each stage to be quite different. You can have as many stages as the Druid has Wildshapes, really. It need not be an actual Druid, just ripping the mechanics of Druids, anything with Wildshape works.

Another is a spell that allows one to fuse a CR 2-3 outsider with a zombie. Fudge the rules slightly and you can have a very low level 2 stage boss fight by having one Ghoul or Ghast that got a contingency casting of the spell to trigger when they hit half health. Now, the thing has the powers of whatever CR 2-3 Outsider you feel like on top of being a Ghoul or Ghast. Works best for very long campaigns as a way of showing that some nasty cheese is to be had. This setup may even be fully RAW legal, I don't recall the exact wording of the spell. Heck, the CR is a guess based on the example of a Lantern Archon.

If you want a degenerating multi-stage boss, then there's a few old ideas I've had for massively powerful Undead/Construct merged atrocities. One of the possible flavors for Golems is bound elementals, and spells exist to bind low-CR Outsiders to Undead already. Focus on the fluff instead of the crunch and you can have something that breaks down as it's damaged, losing the abilities of it's component creatures one at a time. At the absolute lowest end, you have the Zombie/Animated Object. When it runs out of HP on one side, that sides stats and abilities are removed from the total. You are more likely to go with considerably higher end types of Construct and Undead, but the idea is generally that the body is made a Construct and an Undead, stacking the stats.

[Morphic tide]

Let's explore the Zombie/Animated Object example in a bit more detail.

The standard Human Commoner Zombie has the following stats:

Medium Undead
HP: 2d12+3HP
Initiative: -1
Move speed: 30ft.
AC 11, Touch AC 9, Flat Footed AC 11
Base attack +1/Grapple +4
Attack: 1d6+2 Slam, +2 to-hit
Space/Reach: 5ft.
Special attacks: None
Special qualities: Single actions only, DR 5/Slashing, Darkvision 60ft. and Undead traits
Saves: Fort +0, Ref -1, Will +3
Attributes: Str 12, Dex 8, Con -, Int -, Wis 10, Cha 1
Skills: None
Feats: Toughness
CR: 1/2
Alignment: Always Neutral Evil
Advancement: None


The Medium size Animated Object of humanoid shape (two legs, two arms) has the following stats:

Medium Construct
HP: 2d10+20
Initiative: +0
Move speed: 40ft.
AC 14, Touch AC 10, Flat Footed AC 14
Base Attack +1/Grapple +2
Attack: 1d6+1 Slam, +2 to-hit
Full Attack: 1d6+1 Slam, +2 to-hit
Size/Reach: 5ft.
Special Attacks: None
Special qualities: Construct traits, Darkvision 60ft, Low-Light Vision, Hardness (whatever you can say human flesh has)
Saves: Fort +5, Ref +3, Will +0
Attributes: Str 24, Dex 6, Con -, Int -, Wis 1, Cha 1
Skills: None
Feats: None
CR: 7
Alighnment: Always Neutral


Here's what I'd use as the statline for the mix:

Medium (?)
HP: 2d12+2d10+23
Initiative: +0
Move speed: 40ft.
AC 16, Touch AC 10, Flatfooted AC 16
Base attack: +2/Grapple +6
Attack: Slam 2d6+3, +4 to-hit
Full Attack: Slam 2d6+3, +4 to-hit
Size/Reach: 5ft.
Special Attacks: None
Special qualities: Merged Creature (Zombie/Medium Animated Object) Construct traits, Undead traits, Darkvision 60ft, Low-Light vision, DR 5/Slashing and Hardness (whatever human flesh has)
Saves: Fort +5, Ref +4, Will +3
Attributes: Str 26, Dex 8, Con -, Int -, Wis 10, Cha 1
Skills: None
Feats: Toughness
CR: (?)
Alignment: Always Neutral Evil


Merged Creature: This creature is a merger of two different creatures, sharing most of the strengths and some of the weaknesses of the listed creatures. When this creature has taken enough damage to reduce the first listed creature's HP to 0, it becomes the second listed creature. (What do you think of this quality? I could have made it into a template, but that leads to issues involving rules text. Much cleaner to use a quality that can express more esoteric situations, like demonic possession that has the demon manifest once the possessed dies.)

[Nettlekid]

For a Zelda: Twilight Princess kind of tiered boss battle, the psionic Body Leech suits pretty well. The captive hostage is on the wall in the background in plain sight and the party fights the Body Leech with its potent psionics, defeats him, and frees the hostage...who immediately picks up where the Body Leech left off after the Body Leech mind swaps into them.

For something more like a boss going through different forms, I would vote for using layers of Undead. There are Undead that are just skin or just bones, so stick them together into one body! Start with a Forsaken Shell as the skin, which takes the first few hits and breaks away when its HP is gone to reveal phase 2. Underneath that is some monstrous Undead like a Vilewight or Vampire or whatever you want, strong enough to be a threat. It attacks with claws peeling through the Forsaken Shell, so it's fighting the whole while despite the Forsaken Shell taking hits. And finally when that's killed, its flesh falls away to reveal a Spellstitched Skeleton underneath who, while physically weaker, uses magic for a very different fight.

I think you could make use of a Lich. Not in the way you're thinking, not about the Lich's ability to regenerate, but because they have stages of life. For mid-level, imagine the party fighting a Grisgol, a construct fueled by a Lich's phylactery. They defeat it, the Lich respawns, and continues fighting. If you're talking higher level/BBEG level, imagine fighting a regular Lich that uses mid-level magic, and then upon its defeat reveals itself to be a Demilich head that was just sitting on top of a skeleton body, and as the body falls away it begins using its stronger abilities. Bonus points for making the headless skeleton partially sentient so that when it was pretending to be a regular Lich it gets double actions, but doesn't use its paralyzing touch (because the skeleton's hand doesn't have it).

[Flickerdart]

Didn't look in any detail but wouldn't it be possible for a creature with the swallow whole ability to swallow the boss, then fight the group and the real boss breaks free only after defeat of the creature who swallowed the boss. Of course the boss must be immune to the damage done by the swallow whole ability. I know it is a stupid idea but hey, it is creative.

It works, but one must wonder what the point is. The swallowed creature would be better off fighting alongside its pet.

Hypothetically, you could have a rogue that uses Escape Artist to leave the gullet, make a sneak attack, and then on the T-Rex's turn it swallows the rogue again...

[icefractal]

It works, but one must wonder what the point is. The swallowed creature would be better off fighting alongside its pet.If it was a buff-oriented type, being inside the stomach lets it buff/heal the creature with no risk to itself. Or even multiple buff-oriented types, if the monster is big enough. Imagine how nasty a titan would be if it's getting healed every turn, bard-music enhanced, and extra actions from White Raven Tactics or the like.

This would make stage two the lesser one though, as you're fighting foes that aren't very well suited to being on the front-line themselves.

This could be a nasty three stage fight, if someone inside reanimates the corpse as undead right after you kill it the first time.

[Florian]

In today's post, I'd like to talk about the thought of bosses that have "phases". I remember reading something a long while back, an inspired post where someone talked about the idea of slaying a boss, the party would be excited and checking for loot and asking for experience, but then all they would get in response is the boss arising from the floor, stating "I am still here", at which point they have even more abilities at their disposal.

I was wondering, if anybody has done something like this using 3.5e books, how did you go about this? What was the encounter like, and how many phases did the boss have? Aside from homebrewing it up or using some kinda modification of Death Pact, I'm not sure how I'd do it below 16th level.

Not exactly 3.5 but PF, but there is an encounter in an AP that is as memorable (and problematic) as meeting TDC for the first time.

In this case, a Shadow Demon has possessed a powerful ape-shaped host body and uses it to fight with reckless abandon. This is a taxing fight and ocne it is over, the actual enemy, the Shadow Demon, appears and is ready to continue the fight with the next best host body available.

[Flickerdart]

If it was a buff-oriented type, being inside the stomach lets it buff/heal the creature with no risk to itself. Or even multiple buff-oriented types, if the monster is big enough. Imagine how nasty a titan would be if it's getting healed every turn, bard-music enhanced, and extra actions from White Raven Tactics or the like.

This would make stage two the lesser one though, as you're fighting foes that aren't very well suited to being on the front-line themselves.

This could be a nasty three stage fight, if someone inside reanimates the corpse as undead right after you kill it the first time.

Oh, good idea.

Phase 1: Giant monster buffed by a spellcaster inside its gullet. Ends when the monster is slain.
Phase 2: Giant monster is reanimated by the spellcaster. However, the undead (probably) loses Swallow Whole, and now the caster must ride the creature.

[Jack_Simth]

Hmm... maybe combine a few ideas?

A long time ago, there was an angry, powerful ghost/demon. It made enemies of some bad dudes. During one fight with said bad dudes, it possessed a guy.

The guy's buddies solved the problem... by casting Imprison Possessor and Impotent Possessor (both Book of Vile Darkness). Confident that they wouldn't have to worry about it as long as the guy lived, the buddies retired.

Fast forward a bit... said guy isn't very nice. He Mind-controls a nasty monster and rides around inside it, giving it directions and buffing / healing effects as needed. If it dies, he reanimates it as an undead and comes out to join the party. If the guy gets killed, angry ghost/demon is set loose (and is still angry and powerful).

1: Main monster.
2: Controlling Guy + Undead Monster.
3: Demon/Ghost

[gr8artist]

The obvious solution(s) would be DM fiat. Treat the first and second phases as two separate encounters for calculating CR and loot/exp rewards, no different than "As the monster falls, a stronger monster kicks down the door and enters the room).

Something that might be interesting to try is a Wizard or gish wearing armor that protects him (Con bonus as temp HP, continuous spell effect to gain more temp health) but can't effectively cast while wearing it. So when you break through the armor's usefulness, he drops it and proceeds to start casting the big spells.
A barbarian character that doesn't start raging until it's low on health, maybe. In PF, orcs and half orcs have ferocity that lets them take an action when knocked unconscious - entering a rage to regain some HP might be an option there. Contingent cure spell(s), or perhaps a single-use wondrous item that casts a few spells simultaneously when conditions are met (ie HP < 1).

Blatant DM fiat involved, but I ran a quest where the final boss was a demonic creature trapped in a pillar of ice. His ice spells would not only damage players, but restore HP to parts of the pillar that had suffered damage. To offset his poor action economy, he regularly summoned worshipers and cultists. If I were to do the quest again, I'd have the demon become mobile and start flying around when the column broke. He's no longer protected, but he's a greater threat.

[Afgncaap5]

The obvious solution(s) would be DM fiat. Treat the first and second phases as two separate encounters for calculating CR and loot/exp rewards, no different than "As the monster falls, a stronger monster kicks down the door and enters the room).

I think this is the best way, honestly. I'd want to play it in story as if there's something more than "The monster's dead... wait, no he's alive again and stronger!", but I think this is the best way.

I have a boss fight planned (that, sadly, I've never been able to drop into a story) with a wizard who's controlling an army of goblins and trolls who's having them overpower the countryside with something called "Titan Draught" that he's using to turn his minions into giant monsters. In the final fight against him, he has a set number of spells that he's willing to cast and a set amount of hit points he's willing to lose (effectively, getting to 0 hit points or lower is his cue to run rather than the point at which he's knocked out or killed.) The fight against him takes place at the top of his tower, much of the center of which he's hollowed out to act as a permanently churning vat for constant production of Titan Draught. If the players get him to the point where he's done casting spells or out of hit points, he'll make a run for the pit inside his tower, dive in, and completely submerge himself in the draught. That's when I swap out his first character sheet for his Final Form true sheet where he grows to the size of a truly titanic monster, with his arms and upper torso and head sticking out of the pit. Then he'll start casting the other spells he's got prepared (which will be stronger because he's bigger, something which I believe actually violates the rules of the game but I'm okay with), trying to swat players away (it's amazing what counts as a "threatened square" with the arms he's got) and occasionally trying to shake the top of the tower down onto the players since, he reasons, he's probably large enough to survive the roof collapsing.

Granted, I'd have to figure out how he can get to the tower easily without the players stopping him (something offering a freedom of movement effect, probably) and what's to prevent the players from just diving headfirst into the titan draught themselves (even if I declare it poisonous and put a force field up it's amazing what a dedicated player with a dumb idea can do), but it's a scene I've always wanted to get to.

Now, what I *have* done is making this work for weak monsters. I modified the Shadow Sibling from Eberron to be a kind of possessing spirit infecting a lot of seemingly insane villagers (mostly level 1 Commoners but with a level 3 Warrior in for good measure) against some starting players who were tasked with carting them between villages (they didn't know it, but they were basically going to go to an insane asylum.) When the "madness" spread to the person driving the wagon from the front, it crashed and players had to fight people who'd been modified with the Shadow Sibling effect. Whenever they killed or knocked out one of the villagers, it'd collapse, but their feint outline would still be standing up, meaning the players would then have to fight the creature that'd been possessing them (which I treated as the Shadow Sibling stats, but I changed the creature type to Undead.)

[adonis53]

You don't really need a mechanically correct way to do it though... I quite literally threw a single adamantine golem at my players which, once killed, released a slightly smaller Mithril golem who was much faster then the former golem, forcing the party to completely change their strategy.

TL:DR: No mechanics needed, just explain it in roleplay if it's fun. #TheRuleOfCool

[Sheogoroth]

I see boss fights as my opportunity to go nuts and break the rules.
I rarely do any tweaking to the base stats or abilities of monsters, but boss fights should ALWAYS be surprising. Sometimes I'll even break the turn order to give them a last grasping, sputtering curse.

I'm having my characters face off against a demon worshiper who makes a rather important sacrifice to be transformed into a demon just as the players enter.

I gave her a series of transformations(claws, wings, mandibles, plated skin, extra limbs, etc.) at various intervals.
Finally by the end of the fight she just devolves into this wriggling mass of fangs and limbs before they put her down.

Just make stuff up, you're the DM. Mechanics are there to help you, but they can hinder you if you can't move beyond them.

[gr8artist]

PF's Synthesist (http://www.d20pfsrd.com/classes/base-classes/summoner/archetypes/paizo---summoner-archetypes/synthesist) Summoner (http://www.d20pfsrd.com/classes/base-classes/summoner) can conjure an Eidolon (extraplanar customizable pet, like a sup'd up animal companion) that wraps around his body like armor. Have him do this when reduced to low HP, giving him the Eidolon's attacks and special abilities.

[Morphic tide]

I guess I'll give some more coherent setups, based on what's been brought up in-thread so far and what I've though up just now:

Synthesist Summoner: Actually works a bit better if you use the Eidolon at the start, they fight in melee as a rather effective close combat enemy, then they switch to standard minionmancy once the Eidolon drops. This way, you aren't doing awkward DM fiat(bringing out the Eidolon takes 10-100 rounds, because it's measured in minutes) and the boss is doing things more or less properly.

Tactics-based: Listed separate from above because the above is just normal gameplay for Synthesists. Basically, they start the fight acting as a BFC-gish, using Polymorphs, buff spells and so on at the start and then switching to the save-or-lose/suck or minionmancy when they get close to dying. Works well as either Druid or Psion, as Druids can function well in melee without burning spells a lot and Psions have the ability to use exactly all the Powers you need them to, if they are a BBEG leading a group of Psions. Power sharing, for everyone!

DM Fiat: Great for roleplaying heavy games, but it reeks of lazy storytelling in highly optimized games because you really should be able to optimize your way into almost any situation you need without needing DM fiat. I mean, look at all the other options in this post!

Contingency: Animate Dead, strong buffs, Polymorph, Summon spells and more can all be given a contingency of "on death/low HP" so that the low-level boss becomes a higher-level encounter upon death or low health. Of note is that quite a few Undead templates exist, as well as stuff to stack on Summoned creatures. You can also have the Undead raising be in an area, making the group of enemies slain previously come back.

Homebrew: Very similar to DM fiat, but you have codified rules governing it and might as well let the players use it, so they don't call foul when you start pulling frustrating stuff. Even more fun for some, the players can copy the multi stage fight trick you set up.

[ATHATH]

I know that someone posted the first one of these here earlier, but I recommend also checking out the follow-up articles:
http://theangrygm.com/return-of-the-son-of-the-dd-boss-fight-now-in-5e/
http://theangrygm.com/elemental-boogaloo/
http://theangrygm.com/oh-no-more-bosses-oozes-slimes-and-a-duplicating-wizard/

[Swaoeaeieu]

Not really the type of boss survival bossfight you are talking about, but a cool idea i have yet to try in my group:
I remember a campaign journal where the group was attacked by a group of 10 or so big spiders. Every time they killed one, the rest got bigger and stronger, even more intelligent. Untill the final spider turned into a drider (am i spelling that right?) casting spells and shouting obsceneties at the partie in some spider language.

seems like a cool way to keep changing the fight, slowly turning it into a bossfight along the way.