Monday, October 26, 2009

Descending AC is Actually Rather Neat

[I recently read about how it's easier to calculate small even numbers, use only additions and compare vs 20 ( a smallish even number ). Lost reference, sorry. It caused me to coalesce these d20ish resolution mechanic.]

Basic Idea
  • d20 roll
  • natural 1 is automatic fail, no matter what always a 5% risk
  • natural 20 "explodes" i.e. add d12-1 to result (a subsequent roll of 12 explodes as well, recursively, but has no practical benefit)
  • modifiers should be positive additions (not subtracted, not negative)
  • >= 20 is success
  • 21-29 increasing range of success (if applicable)
  • >= 30 is spectacular success

To Hit

d20 + ToHit bonus(es) + Defense >= 20 is a hit. >= 30 is critical hit.

ToHit Bonuses are class/level base bonus, +1 magic sword, +2 flank attack, etc.

Defense is opponents descending AC. The best possible AC is 0. Heavy armor and huge shield is 0 not -2.
  • No armor (def 10) hit on 10-20 55% chance of hit
  • Med armor (def 6) hit on 14-20 35% chance of hit
  • Max (def 0) hit on 20 5% chance of hit
  • +2 to hit vs common med armor/shield (def 2) hit on 16-20 25%.

The way the d20 explodes and crits are determined means less armor and/or more tohit bonuses (mostly BAB) equates to increase chance of crits. Which is exactly right IMHO. I'm currently interested in play below level 9 with little bonus inflation. So, the AC cap doesn't bother me much. It forces me to create monsters that have interesting challenges rather than just more AC and more hit points.

Skill/Ability Checks

Using C&C SIEGE engine all skills and abilities are checks using one of the six abilities. Note: rolls should be used only for exceptional circumstances. Common skill/ability use should just succeed. Also, most rolls aren't success/fail but rather degrees of success/complication. Cause falling to your death every 20 checks on average gets old.
Example: Climbing a rope -> no check. Climbing a rope that is on fire while crazed monkeys jump up and down on your heads -> roll to see if you make it to the bottom before rope burns through. Fail -> monkey has stolen your equipment or rope breaks and you grab a tree branch from which you are now pecariously hanging or hahaha you fall to your death roll new character, no you can't keep your old charsheet it goes into the shredder like all the others!

d20 + StatMod + LvlBonus + Difficulty >= 20 success. >= 30 is critical success.

StatMod is std +1 to +3 for high abilities (oh, I guess there are negatives here with low abilities) and +4 more if ability is prime.

LvlBonus is a tricky one. Straight up adding level is too much (8th lvl prime is +12 to +15 which is 95% success for most challenges) No bonus means characters never get better. Guess I'm going with 1/2 level which seems fiddly to me. I'll probably create a couple scales 1/2, 1/3 1/4 (like tohit has) for different "classes" (pie pieces actually)

Difficulty (descender) moderate +10, hard +5, heroic +0. Easy tasks just happen no roll is needed. Alternatively, any number 10 to 0.


Using C&C SIEGE engine all saves are checks using one of the six abilities. Saves should be hard. 10% (30% if prime ability) success rate. Spells cast by higher level wizards, deadlier poisons, ancient dragon breath, etc should be harder to save against than weaker threats. Saves should become easier as characters gain experience. This implies something like CharLvl - ThreatLvl or multiple target numbers instead of just '20'. Or increasing save threshold to 30.

d20 + StatMod + LvlBonus >= 20-30+ is Success Magnitude.

StatMod is std +1 to +3 for high abilities (oh, I guess there are negatives here for low stats) and +4 more if ability is prime.

LvlBonus + 1 per level

Success Magnitude -20 must be >= to threat level i.e. 8HD Shelob's poison requires 28 or better to save against.

So, equal lvl threats can be saved against 5% (25% prime) I guess that works.

I'm most dissatisfied with saves... I'm half inclined to throw out varying difficulty and increasing save chance with character level. Just have static flat 5-40% chance to save based solely on prime and stat mod.

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