Spell Damage is a modifier stat on many higher-level pieces of equipment. This stat helps direct damage spells deal more damage and is useful for offensive spellcasters and most hybrids.
Mechanics[]
Spell Damage increases the damage of direct damage spells according to the following formula:
(Cast_Time + Recast_Time) / 7 * Spell_Damage = bonus damage per hit of spell
Where Cast_Time is the spell's base casting time in seconds, Recast_Time is the spell's base recast timer in seconds, and Spell_Damage is the caster's total Spell Damage stat.
This bonus is applied on every hit of the spell, meaning that AoE spells and rains will gain additional benefit from it due to their multi-hit capability.
Damage added as a result of Spell Damage is a flat bonus tacked on at the end of other calculations. In other words, this damage cannot crit and is not affected by focus effects.
The stat does not affect DoT spells in any way.
Strategic Impact[]
By far, the spells that benefit the most from Spell Damage are AoEs with significant cast times and recast timers. Due to having higher cast and recast times, these spells gain a larger multiplier for Spell Damage than other magic does. This higher multiplier can then be applied multiple times in a single cast of the spell, due to the spells' ability to strike multiple targets (or, in the case of rains, fewer targets in multiple waves). At high levels, the Spell Damage stat can end up increasing the total damage dealt by such spells by 15% or more. Considering that most such spells cannot be boosted by focus effects, this is a fairly significant increase in damage potential.
Most other direct damage magic receives middling to poor benefits from the stat, which can make it difficult to argue for when attempting to optimize an offensive spellcaster's equipment. Wizards and magicians derive more obvious benefit from it than most others do, due to using direct damage spells (as opposed to DoTs) almost exclusively and due to them possessing a variety of AoE damage spells that are strong, efficient, and see frequent use in circumstances that allow them.
A case can also be made for Spell Damage being a strong stat for beastlords on a burn, since their Maelstrom spells fire multiple direct damage spells at a target with a single cast (and each of the triggered spells has within its spell data a recast time of at least seven seconds, meaning that it gets at least a 1:1 conversion ratio from the stat on each spell that gets triggered by the Maelstrom) and can be used back-to-back with the class's other single-target damage spell lines with long recast times.
Due to the way the math works, spells with both a cast time of zero and a recast time of zero (most damaging procs, for instance) will derive no benefit from Spell Damage. This generally makes optimizing spell damage unimpressive for adding damage to procs. While a few exceptions to this do exist (weapons which proc spells actually castable by player classes, which therefore have a cast and recast time built into their spell data), most of them are of too low a level to make Spell Damage at all relevant.