Help with altering d20 mechanic for old school dungeon crawl

[vgunn]

Here is what I would like to do:

* Classic six ability scores ranging from 3-18
* Roll high (d20) add ability score. Meet or beat DC to succeed. DC starts at 20 and has +5 increments (20, 25, 30, 35 ...)
* Class is similar to careers in Barbarians of Lemuria (no skills, but you can have broad talents or moves)
* Saving Throws tied to the six abilities for reactive checks
* Die-step rather than +/- modifiers
* AC soaks damage
* Hit Points represent actual "hits" you can take before CON damage occurs
* Ability damage (scores can be temp. reduced from different types of attacks)

I'm trying to keep rolls to two dice. So for example, if you are a first level Thief and you are attempting something within your class you roll a d20 + d4 (level) + ability score. Meet/beat the DC to succeed.

If you have a talent or move you can bump up the die. So the thief has a special move and the d4 gets bumped up to a d6.

Opponents/Minions with certain abilities can reduce the level die--but I'd like to try to keep the player to roll just 2 dice.

Class/Level die:

Level 1-3 = d4
Level 3-6 = d6
Level 7-9 = d8
Level 10-12 = d10
Level 13-15 = d12
Level 16-18 = d20

If you want to attempt something outside of your class the second die becomes a penalty. The size of the penalty die depends on the situation.

I hope this is a decent start. But tell me what you think.

Thanks!

[Grod_The_Giant]

This is... brainstorming a full homebrew system. You're aware of that, right?

[vgunn]

This is... brainstorming a full homebrew system. You're aware of that, right?

Yes, noticed it was in the wrong section. Hopefully it will get moved!

[Grod_The_Giant]

I was referring more to the amount of work this is likely to be. You're not rebuilding classes or making minor tweaks to the rules; you're radically changing the most fundamental system math.

On that note, 1d20+1d4+ability modifier will have an average of 14-17, making it difficult even for a very competent character to hit your base DC at first level. Even for your "special moves," 1 die step is an average improvement of +1. I'd look more at adding extra level dice, which

provides a bigger bonus
feels more satisfying (roll all the dice!)
gives you more of a bell curve, as opposed to the massively variable 2d20.

[vgunn]

I was referring more to the amount of work this is likely to be. You're not rebuilding classes or making minor tweaks to the rules; you're radically changing the most fundamental system math.

On that note, 1d20+1d4+ability modifier will have an average of 14-17, making it difficult even for a very competent character to hit your base DC at first level. Even for your "special moves," 1 die step is an average improvement of +1. I'd look more at adding extra level dice, which

provides a bigger bonus
feels more satisfying (roll all the dice!)
gives you more of a bell curve, as opposed to the massively variable 2d20.


Thanks!

What scale would you recommend for the extra level dice?

[Lord Torath]

Yes, noticed it was in the wrong section. Hopefully it will get moved!You can always report your own thread to a Mod (It's the red and white triangle with an exclamation point at the bottom left of your post). :smallwink:

[Knaight]

Thanks!

What scale would you recommend for the extra level dice?

Adding a bunch of d6 can work well. I've crunched numbers on this in one of the 5e threads, so I'll see if I can pull them up.

EDIT: Found it. It was archived, so formatting was lost, but here's what I had. It's within the context of a different thread, so I stripped out most of it.
Performance gets more consistent the better one is - a complete novice demonstrates the ends of their ability far more often than someone who is really good. To simulate that, one could just add multiple skill dice together. Unskilled would be 1d20, then it is just +1d6 per skill point, perhaps to a maximum of +5d6. 1d20+5d6 can theoretically go as low as 6, meaning that all but the very easiest DCs can still be failed, but that this will only rarely happen. At the same time, DC 50 is theoretically possible as well, with this being exceptionally rare. The average is 28, which is pretty solid, but there is still a reasonable chance of failing various tasks. Specifically:

DC 5: 0% Failure Chance
DC 10: 0.3% Failure Chance
DC 15: 2.95% Failure Chance
DC 20: 15.45% Failure Chance
DC 25: 37.64% Failure Chance
DC 30: 62.36% Failure Chance
DC 35: 84.55% Failure Chance
DC 40: 97.05% Failure Chance

This leaves all DCs over 20 beyond the untrained, which seems pretty reasonable. It also leaves obvious room for a skill training feat that doesn't suck: Bumping all of those d6 up to d8 is entirely reasonable. For that matter, if all DCs are kept in the 5-25 range, even the best have a reasonable chance of failing the hardest, without it being guaranteed. Again, this is reasonable. Expanding the math further, this time using a success rate:

Roll|DC 5|DC 10|DC 15|DC 20|DC 25
1d20|80%|55%|30%|5%|0%
1d20+1d6|95%|72.5%|47.5%|22.5%|2.5%
1d20+2d6|99.5%|88.6%|65%|40%|15.6%
1d20+3d6|99.98%|97.1%|81.7%|57.5%|32.5%
1d20+4d6|100%|99.5%|93.1%|74.5%|50%
1d20+5d6|100%|99.95%|98.2%|88.0%|67.2%

That looks fine to me, though I'd be tempted to cut off the +5d6 and instead cap things at +4d6, particularly as attribute bonuses do play into this to some extent. Plus, that nicely works out to the lowest level having all but the highest possible and the highest level having all but the lowest possible, with all of them up in the air for everything else. It also has 5 difficulties and 5 skill levels, which provides some symmetry for those who care about it.