Spell resistance

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, spell immunity, then spell resistance. That is, a spell that does not check spell resistance is not affected by any of these defenses. In addition, using a feat to cast a spell bypasses these defenses, even if the spell would normally check spell resistance (in fact, even if the script appears to invoke a spell resistance check).

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). 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

Thus the maximum possible roll is 66, meaning a spell resistance of 67 renders a creature immune to spells that check spell resistance, regardless of the caster.

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.

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, area of effect and other persistent spells may fail to register as the correct spell in the spell resistance check.

Sources

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.)

Lowering spell resistance is a result of nature's balance (-3 to -32) and the breach line of spells — lesser spell breach (-3), greater spell breach (-5), and Mordenkainen's disjunction (-10).

Custom Content Notes

When the spell resistance check is used inside an area of effect script, it must be preceded (somewhere in the same script) by SignalEvent() where a spell is signaled as being cast in a hostile manner. If this is not performed the check will automatically return 0 and a meaningless spell resistance roll will be displayed if the target has spell resistance (though no mantles will have levels depleted). This peculiarity is limited area of effects and is likely a bug.