Chad Perrin: SOB

26 June 2008

Making Combat Better with the 20+Nd9 Critical System

Filed under: RPG — apotheon @ 11:06

This is part of my RPG series of entries here at SOB. See the inaugural entry in the series for more details.

In my previous essay, Open Ended Rolls, I promised I would describe the open-ended rolling system I’ll be using for the forseeable future in Pathfinder games. This is where that description happens.

It’s more than just a system for open ended conflict resolution rolls, however. It’s also a critical hit system designed specifically to improve upon the vitality and wounds damage system I mentioned I was adopting in Damage Systems in D&D and Pathfinder.

Before I explain the new system in particular, though, I’ll set the stage with references to a couple of other systems I’ve developed.

The Nd6 System

Back in the early ’90s, as I explained in Open Ended Rolls, I developed a system that used multiple d6 dice. That system was designed with the specific workings of the AD&D 2E system in mind. I created house rule after house rule for that system until the game I ended up running in my Serpent’s Spine gameworld wasn’t AD&D at all — it was a separate game of my own creation that came into being incrementally over time.

If one knew the history of how this new game came into existence, the evolutionary line of descent from AD&D 2E to whatever the new system was called was obvious, even if seeing it as a whole system without having experienced the process of mutation might not give you many helpful clues.

Part of the process of creating this system involved reworking the Proficiency system (what you 3E kids would call a Skill system). Back in the day, there were two types of skills — Weapon Proficiencies and Nonweapon Proficiencies. It was the Weapon Proficiencies that really led to the major changes in the game system that, in turn, left me with an Nd6 system. The way the Weapon Proficiencies in 2E originally worked was kind of arbitrary and inelegant: you rolled d20 to attack, of course, but had bonuses or penalties depending on your proficiency with the weapon you used. If you were not proficient in the weapon, you had a penalty to hit, and how much of a penalty was imposed depended on your character class (Fighters had lower nonproficiency penalties than Wizards, naturally). With proficiency, you had no bonuses or penalties — but you might be limited to specific weapons in which you could become proficient by your character class (Wizards couldn’t be proficient in two-handed swords). Spending two Weapon Proficiency “slots” on a single weapon made you Specialized, a level of skill only warrior classes could achieve, and that gave you a +1 to hit and a +2 to damage with that weapon.

The Nonweapon Proficiency was in some ways similar, but in some ways different. Most Nonweapon Proficiencies cost one proficiency slot to become proficient, but some cost two (or even three, in the case of Weaponsmithing), and cross-class proficiencies cost one more slot in addition to the standard cost. Additional slots could be spent on a Nonweapon Proficiency a character already had to increase the level of that proficiency by one point. Proficiency ranks were added to the associated attribute (not the attribute bonus — the whole attribute) to determine the proficiency level and, to resolve an action, the player would roll 1d20 and attempt to get a number under the total proficiency level.

What I ultimately ended up doing was unifying the die roll system for the entire game. Nonproficient characters — whether attempting a task with a Nonweapon Proficiency or an attack with a Weapon Proficiency — would roll 2d6. The standard target number was 10, 15, or 20, depending on the difficulty of the task, but could be further modified by situational bonuses or penalties. For attack rolls, the target was AC, which was the target’s Dexterity attribute (the whole attribute, not a modifier) plus bonuses for armor and magic. If a character had a relevant proficiency (Equestrian for difficult horse riding maneuvers, or Bastard Sword for attacking with a bastard sword, naturally), the player would roll 3d6 instead. If the target number for the task difficulty was higher than the number available from a raw Nd6 die roll allowed by proficiency level, of course, the player needed to hope for some 1 results on dice so that additional dice could be rolled and added into the total, incrementally improving the chances of success. If all dice came up with a 1 result on the initial roll, however, that was a critical failure. As I explained in Open Ended Rolls, a critical success (resulting in double damage, in the case of an attack roll) came about if the total roll was at least twice the target number. Three times as much meant another “level” of critical success (three times the target number meant triple damage, for instance, and so on).

Additional levels of expertise in a given proficiency were possible as well. A fourth d6 could be added to the roll by adding another proficiency slot to the pile, and a fifth, and so on. The levels of proficiency from two dice on up were dubbed Nonproficient, Proficient, Specialized, Master, Mastery 1, Mastery 2, and so on. Notice the elegance of the probability curve’s shape, there: the more dice you roll, the lower the likelihood all your dice will come up with 1 results, but the more likely you are to get one or more 1 results from those dice. Ever-greater levels of proficiency would also significantly improve your chances of killing an opponent in a single strike in combat, as multiples of basic damage are achieved by reaching multiples of the target’s AC, which fits well with the notion that the character is better at this combat stuff.

Of course, with the completely reworked combat and skills system of 3E, this was no longer an appropriate way to roll dice for conflict resolution.

The 2d9 System

Now that the combat and skills system had changed so drastically from the 2E days, I needed to start over on how best to fix the shortcomings of the system. It occurred to me that the best way to do that involved the traditional 0-9 numbering of the d10. I’d simply start calling it a “d9” for conflict resolution rolls instead, and use two of them for standard skill and attack rolls.

When a roll came up all zeros, it would be a critical failure. When it came up all nines, the roll would open up at the high end; for every 9 result on a die, the player would get to roll another d9 and add it to the total.

The fact that the traditional d10 was numbered from 0-9, instead of 1-10, made it easy to treat it as a 0-9 roll. Because the low end of the roll is a 0 which, added to another roll, doesn’t change its value at all, that means that the roll can be open-ended when one rolls the maximum on a die rather than the minimum without giving up the smooth progression of your probability curve — especially when the initial roll is more than one die. Thus, a 2d9 roll +1d9 for every 9 that is rolled provides a nice, fairly natural-looking curve for an open-ended roll, with a “long tail” of probability at the high end of the curve that approaches infinity.

Handling critical successes and failures is another matter entirely. The first instinct, after having developed the Nd6 system described above, was to say that all dice coming up with a 0 would be a critical failure. Unfortunately, that doesn’t reduce the chances of getting a critical failure as a character got better and better at that task — which is one of my major pet peeves with the flat probability curve and “1 is always a fumble” approach of the standard d20 system. The first instinct for determining critical successes, meanwhile, was to go with the “double the target number, double the effect” system — which works great with hit points when resolving attacks.

For critical successes, then, I just decided that with all modifiers (bonuses for magic items, increasing base attack bonus, et cetera) and all additional d9s rolled when a 9 is achieved on a die you just needed to get double the target number. Triple would do triple damage or triple effect (in the case of non-damaging tasks), et cetera.

For critical failures, however, I came up with an arithmetic solution. If your roll ultimately ended up being ten or more lower than the target number, you achieved a critical failure. For twenty or more lower, you doubled the significance of that failure, and for thirty or more lower, you tripled it. How that would work out in terms of actual effects depended on the specific action taken (and would be largely up to the GM — who could, if desired, work up some critical failure charts).

It seemed like a pretty good system, and may still be a good system for a game like Elements Eight. This is the approach I suggested to Shackleton he might want to consider when I said:

Don’t decide on a system yet รข?? you may find that you like the additional system I’m going to describe

Then, I chose to incorporate another variant rule into my games, which threw a monkey wrench into the works — and, ultimately, led me to a system for conflict resolution with open ended rolls that better preserved the flavor of the D&D/Pathfinder system while still giving me the improvements I wanted.

The Vitality/Wounds System

When I added the vitality/wounds damage system, as I mentioned I was going to use in Damage Systems in D&D and Pathfinder, I realized I had to figure out how to deal with critical hits in combat differently. Rather than using a steadily climbing scale of additional damage multipliers, I needed a way to determine when damage should be assigned to wounds rather than vitality.

The way the vitality/wounds system works is actually pretty simple:

  1. Calculate hit points for a character as usual, except now you call them Vitality. Under normal circumstances, you take damage to your vitality, which then heals at a rate of one point per character level per hour. This represents not actual serious injury, but the cumulative effects of exertion and minor cuts and bruises, which can be shaken off relatively quickly. I don’t recall off the top of my head how the book does it, but the way I handle things, your vitality heals at that rate regardless of whether your wounds have recovered — though I’m considering limiting your vitality to a percentage of the total equal to your current wounds capacity’s percentage of your maximum wounds capacity, or the actual vitality to the actual current wounds capacity unless all wounds are healed, or something like that.

  2. A character’s wounds capacity is equal to his or her Constitution score. If you take damage to wounds, you immediately suffer the effects of being “fatigued”. That means you cannot run or charge, and you take a -2 penalty to Strength and Dexterity until you have rested for eight hours or your wounds are healed (whichever comes first). I ignore the “rested 8 hours” part of that, and just go with taking all those penalties until your wounds are healed.

  3. Under normal circumstances, you only take damage to wounds when you’ve run out of vitality. In other words, once you run out of vitality, you start taking damage to wounds.

  4. When you take a critical hit, however, the damage goes directly to wounds. Obviously, the whole “double damage” thing gets ignored entirely.

The way Unearthed Arcana suggests you deal with critical hits, then, is that you translate the damage multiplier for a weapon into a modifier to the critical threat target for that weapon. For instance, with a light mace you get a critical threat target of 20 (normal); with a short sword you get a threat target of 19 (same as in the book); for something with a critical damage multiplier of x3 (higher than normal by one) and a threat target of 20, you change the threat target to a 19 (normal, reduced by the difference between that weapon’s damage multiplier and the multiplier for a “normal” weapon).

I don’t like the way that works out, so I came up with my own ideas for a variant.

The 20+Nd9 System

Instead of using 2d9 for attacks, I went back to 1d20. The flat curve seems bothersome at first, but with an open ended roll that starts creating an actual, non-flat curve again once you get beyond the 1-20 range, all that really matters is the statistical probabilities — not really how they look when graphed. Whenever a 20 is rolled, add a d9 to the roll, though, thus creating an open ended roll.

Thus, when a target number is doubled, you get a critical hit and damage is shifted from vitality to wounds. For critical failures, you can use the “confirm the effect” roll that D&D suggests for critical results; roll another d20 (with all the added d9 the roll calls for, naturally), and consider it a confirmed critical failure if you come in under the target number on the confirmation roll. In both cases, all relevant modifiers are applied.

This allows for increasing chances of critical successes and decreasing chances of critical failures as skill improves. It even fits the flavor of a 3E/Pathfinder game better than any of the other options I’ve come up with (or seen elsewhere), and is quite easy to manage in terms of bookkeeping as well as in terms of rolling and adding up dice. If you roll a single d9 to add to a d20 roll, you’re actually doing less bookkeeping and arithmetic than if you use the standard “rolled a 20, now roll to confirm” system, because you only have to add all relevant modifiers to a roll one time — and if you need an extra d9 or two added to the roll because you keep rolling 9, the tension of the situation certainly makes the very minor extra effort involved worthwhile.

It’s not like you’re going to roll 17d9 on every attack, after all.

All that remains is figuring out how to handle those critical numbers for the various weapons. Let’s take a military pistol from the Iron Kingdoms gameworld book as an example: its critical hit statistics are 19-20/x3. Since we’re not using damage multipliers for critical hits, and we’re not using raw d20 roll results for critical hit determination, we have to do something else with those numbers — or just ignore them. There are a number of reasonable options open to us:

  1. Use the critical threat range as provided for by the Unearthed Arcana rules to determine when the first d9 is rolled and added to the total on a d20 attack. For instance, the military pistol would allow you to roll a d9 and add it to the total if your raw roll is in the range of 18-20 (19-20 +1 for the x3 damage multiplier).

  2. Use the standard critical threat range (19-20 in the case of the military pistol from Iron Kingdoms) and use the multiplier (x3 for the military pistol) to determine how many dice are rolled. Count the d20 that was already rolled as the first die, and roll Nd9 where the N added to the 1 from your 1d20 roll yields a sum equal to the multiplier — meaning that a x3 multiplier adds 2d9 to the 1d20 roll.

  3. Use the standard critical threat range, as in the preceding case, but apply a damage modifier determined by the multiplier even though damage is assigned to wounds instead of vitality. In this case, I’d say add the multiplier number, minus two, to the damage roll. Since a military pistol does 2d6 damage and offers 19-20/x3 as its critical hit stats, you would roll an additional d9 die when you roll 19 or 20 on d20 and add it to the total, roll additional d9s if you roll 9 on your d9 dice to give you an open ended roll, and if your total comes out to two times the target’s AC (or more) you do 2d6+1 damage (that’s 2d6 damage, +3, -2).

  4. Use the multiplier to indicate how many times a +1 is added to damage if your character is proficient in the weapon in question. This adds to the lethality of proficient characters as contrasted with nonproficient characters (such as a warrior vs. a wizard using a martial weapon). Treat the threat and critical system otherwise exactly the same as in the preceding example. The way this would hash out is that a proficient character always gets a +1 to damage (2d6+1 in the case of a military pistol), an additional +1 when a critical hit is achieved by rolling double the target’s AC (2d6+2 to wounds for the military pistol), and in the case of a multiplier of x3, another +1 if you roll triple the target’s AC.

  5. Use the threat range as in the preceding couple examples, but use the multiplier as the penalty to Strength and Dexterity for wounded characters, instead of the standard -2. Thus, with a x3 modifier, wounding a target with a critical hit would impose a -3 to Strength and Dexterity until the wounds are healed, instead of the standard -2 for being fatigued.

There are other possibilities as well, but of those that have occurred to me, these strike me as the best options. I think a certain amount of playtesting is required to really arrive at a definite favorite — and the choice may actually vary depending on the intended flavor of your particular campaign, in addition to depending on things like not making your PCs feel robbed of the extra coolness of an additional damage multiplier or killing off your PCs too easily if they get hit by an orc with a greataxe (1d12, 20/x3).

I still need to decide what I’m going to use in my ongoing game, a session of which — by the way — I’m running this evening. Maybe I’ll ask the players what they think.


Feel free to contact me about compensation if you decide to use any of my game system ideas in a game you’re publishing — either professionally or via amateur distribution. At least give me some game development credit commensurate with the importance of what you use to your game system.

As always, the content of this site is distributable under the terms of the license indicated at the bottom of the page (as of this writing, the PDL).

1 Comment

  1. […] that set of rules is probably wholly inappropriate for that adventure path. On the other hand, some critical hit rules I cooked up should be able to fit in quite well, if the GM’s amenable to […]

    Pingback by Chad Perrin: SOB » The Hacker GM — 11 February 2009 @ 10:31

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