Spell Design: Depletable Statistics

Tuesday, February 8, 2011

We're just about ready to send Ultimate Magic off to the printer! The last chapter is a big collection of spells, and Chapter 2 includes a 12-page section about spell design, complete with an analysis of what goes in the spell stat block and benchmarks of good, typical, and poor spells for each spell level. One of the topics discussed in that section is the idea of "depletable statistics"—numbers in a creature's stat block that kill or incapacitate the creature when the number reaches 0. Examples of depletable statistics are hit points, ability scores, or even levels or Hit Dice—knock those down to 0, such as with fireball, poison, or enervation, and the creature is out of the fight. The primary depletable statistic is hit points, of course, and the advantage of attacking hit points is all characters are able to deal hit point damage, so blasting an opponent with a direct damage spell allows the party fighter and rogue to work toward eliminating that opponent.

By comparison, a creature's stat block has many numbers that are not depletable statistics—not that you can't penalize those numbers, but reducing them may not have an adverse effect on the target and won't eventually kill them. For example, a spell that gives a target a –10 attack penalty has little effect on a sorcerer casting fireball, as would a spell that gave her a –10 penalty on her Will saving throw; despite her poor attack rolls and miserable Will saves, she is still quite capable of blasting her opponents to bits, whether these penalties are –10 or –100. Similarly, a fighter with a –10 penalty on Fortitude saving throws can still swing a sword, as can one with a –10 penalty to Armor Class; the fighter is still viable despite these penalties. These enemies may be vulnerable to other attacks because of these penalties (the sorcerer with a –10 Will save is a sucker for charm person, and the fighter with a –10 penalty on Fort saves is wary of poisons) but the penalties themselves won't kill them. Attack bonuses, saving throw bonuses, Armor Class, CMD, CMB, initiative, speed, skill modifiers, and most other game statistics aren't depletable statistics. This is not to say that targeting these numbers is a bad idea—a brute monster with a –20 penalty on its attack roll is no longer a threat and easy to dispatch—but doing so doesn't have a reachable goal of disabling that opponent with these penalties. Furthermore, spells that penalize these statistics generally don't stack with themselves; multiple castings of bane don't result in greater attack penalties, multiple castings of slow don't make enemies immobile or unable to take actions, and so on.

The idea of a game stat being a depletable statistic or not is a helpful concept when you're comparing the power levels of two spells. Because balancing spells lacks the formulae of pricing magic items, comparison is the only way to judge whether or not a new spell is at the appropriate spell level, and in most cases it comes down to which spell is more effective at disabling an opponent, often by targeting depletable statistics.

Sean K Reynolds
Developer