- This article is about the effect and item property; for the spell, see spell resistance (spell).
Spell resistance (SR) is a special defense against some spells, akin in some ways to armor class. When a spell that checks spell resistance (as indicated in the spell's description) is cast against a creature with spell resistance, there is a chance for the spell to simply not affect that creature based on the creature's numeric spell resistance rating (but it may still affect other creatures, if applicable).
While the numeric rating is the defense called "spell resistance", a spell resistance check actually involves three defenses. The defenses in the order they are checked are spell level absorption (mantles), spell immunity, then spell resistance. That is, a spell that does not check spell resistance is not affected by any of these defenses. Furthermore, spell resistance is usually limited to standard spells, as other abilities usually bypass these defenses, even if the script attempts to test them (the relevant command returns "invalid"). The abilities that bypass spell resistance are generally monster abilities, item powers, and feats—including feats that mimic spells—but more precisely this is controlled by the "UserType" column in
spells.2da (which might not perfectly match player perceptions).
Two abilities that do respect spell resistance are hell inferno and planar rift.
The third part of a spell resistance check — the part called "spell resistance" — is a die roll. Each creature has a numeric spell resistance rating, and if this rating is positive, the caster rolls a d20. To this is added the caster level and the modifier from spell penetration, greater spell penetration, or epic spell penetration (+2, +4, or +6, respectively). For this check, special abilities use a caster level of 2×innate level − 1 instead of the caster level specified in the Toolset. If the modified die roll is less than the spell resistance rating, the spell does not affect that creature.
d20 + caster level + spell penetration vs. SR
For a level 40 caster, the maximum possible roll is 66, meaning a spell resistance of 67 renders a creature immune to PC-cast spells that check spell resistance.
One peculiarity of the order of these checks is that spell level absorption is checked first, so spell mantles can be stripped away by spells that would have been stopped by spell immunity or spell resistance. While this is a suboptimal usage of defenses, it might in fact be the intended correct behavior. (A mantle is described as a barrier around a creature, while immunity and spell resistance are often properties of the creature. Hence a spell would have to get through the mantle before it could interact with spell resistance and immunity.)
Due to game limitations, spell resistance checks try to resist the most recently cast spell of the caster (or creature causing the spell resistance save as in planar rift). As a result, persistent spells (those that can initiate effects after some time has passed) may fail to register as the correct spell in the spell resistance check.
Spell resistance can come from items, spells, or a monk's diamond soul feat. It can be reduced (unless from diamond soul) by certain spells, but not below zero (the full reduction is remembered, though, in the event that higher spell resistance is obtained before the reduction expires). Neither spell resistance nor spell resistance reductions stack; only the greatest of each is used.
Item property-based spell resistance is available from 10 to 32, odd numbers excluded.
Spell-based spell resistance comes from the spell called spell resistance, which confers spell resistance of 12 + caster level, hence a spell resistance from 21 to 52.
Diamond soul gives spell resistance equal to 10 + monk level + improved spell resistance, the total of which can range from 22 to 70. (In particular, taking improved spell resistance multiple times can give a high-level monk a spell resistance of 67+, allowing the monk to resist spells from any caster.)