Difficulty class

The Difficulty Class is a number associated with an action that represents how hard that action is to do. When the game engine checks to see if an action succeeds, it will roll a d20 (i.e., it will generate a random number between 1 and 20), and add any relevant modifiers to that number. If the resulting number meets or exceeds the difficulty class of the action, the action succeeds. A very common example is when a character must perform a saving throw, so that is what we will use as our example.

Example
Imagine that ArgleBargle, the charming and dashing wizard of some renown (in his own mind, at least), had been too busy rooting through his pockets in search of candy to notice that the rogue in his party had stopped in his tracks and begun gesturing frantically that he has spotted a trap. Marching past his desperate friend, ArgleBargle triumphantly produces a gobstopper, and steps on a pressure plate. A loud "click" is heard, and, with a puzzled expression on his face, ArgleBargle finds himself regarding the dart that now sticks out of his posterior. The game engine then checks to see if ArgleBargle successfully performs a saving throw vs. poison.

This is a mild poison, with a Difficulty Class of 10. The game engine will roll a number between 1-20, and then add ArgleBargle's saving throw modifier to that number. If the resulting number is 10 or above, ArgleBargle will walk on unscathed. If the resulting number is below the poison's Difficulty Class (10 in this case) OR if a natural 1 is rolled then ArgleBargle will have failed his saving throw vs poison, and suffer a nasty itch that will last for weeks.

DC As Applied to Player Spells
The likelihood that your spells will land (their saves) is determined by a few factors:


 * Your Base Casting Stat:
 * Intelligence: Artificers and Wizards
 * Charisma: Sorcerers and Bards
 * Wisdom: Clerics, Favored Souls, Paladins, and Rangers
 * This includes starting stat at creation, any points put into the stat at levels 4, 8, 12, 16, and 20, and corresponding Tomes used.


 * Your Gear
 * The item you wear to boost your casting stat (ex: Intelligence +6 Goggles)
 * Any stacking exceptional bonuses to your casting stat (ex: Shroudcraft, Dragontouched Armor Crafting, Tower of Despair Rings)
 * Spell Focus Items (ex: "Necromancy Focus" or, even better, the Stormreaver's Napkin)


 * Feats
 * The metamagic Heighten Spell, for lower than 9th level spells
 * Spell Focus, Greater Spell Focus and Epic Spell Focus if the spell is in the school specified by the feats.

DC is raised by 1 for every 2 points your main casting stat increases, so it will go up on even stats (check your modifier for the stat to see the total bonus). Regular Spell Focus items will raise spell DCs for the specified school by 1, Greater Spell Focus items will raise spell DCs for the specified class by 2. Heighten will raise the level of the spell to the highest spell level the player can cast. You can view the save for any spell by mousing over the spell and reading the tooltips.

So, in equation form:

10 + Spell Level + Casting Stat Modifier + Items + Feats = DC

Example:

Player A is a level 20 wizard casting Finger of Death, a level 7 Necromancy spell with a Fortitude Save. She has a 40 INT, for a +15 modifier. She has Spell Focus and Greater Spell Focus: Necromancy, for +2 to DCs of her Necromancy spells. She is also wearing a Stormreaver's Napkin, for +1 to her DCs. She is using Heighten Spell to bring the level of the Finger of Death to level 9.

Plugging this into the above equation, we get:

10 + 9 (Spell Level with Heighten) + 15 (Modifier) + 1 (Napkin) + 2 (Feats) = 37

So the mob has to make a 37 Fortitude Save or die.

Spell Penetration also goes hand-in-hand with DC, as you first need to break through the target's Spell Resistance to force the target to make a save for the spell. For more information on Spell Penetration and Spell Resistance, please click the corresponding links.