Difficulty Class (DC) is a number that determines how hard a particular action is. When a character's action has a DC, that action will only be successful if the character rolls a d20 (20-sided die), adds all relevant modifiers, and ends up with a number that is equal to or greater than the DC. If the result is less than the DC, then the action fails. (So the higher the DC, the harder the action.)
One action associated with a DC is the use of a skill. The DCs for these skill checks are determined using various methods, as indicated in each skill's description. Two common methods are opposed checks and explicit formulas. For most other skills, the DC is set (or left as the default) by the module designer.
When a spell allows a saving throw, the DC is determined by the spell level, the caster's primary ability modifier (for the ability on which their spells are based), and the caster's feats, as follows:
- DC = 10 + spell level + ability modifier + feat modifier.
The feat modifier is +2 for spell focus, +4 for greater spell focus and +6 for epic spell focus. The feat modifiers are not cumulative. Spells cast from scrolls do not factor in the caster, instead assuming a standard combined ability/feat modifier of +3; that is, the DC is 13 plus the spell's innate level.
Monster special abilities (selected in the Toolset), if not otherwise specified, have a DC of
- 10 + innate level + charisma modifier.
Abilities given by feats, if not otherwise specified, have a DC of
- 10 + class level + ability modifier.
When a trap or aura allows a saving throw, the DC is set (or left as the default) by the module designer.