Spell resistance (SR) is a special defensive ability that allows a creature or item to resist the effects of spells and spell-like abilities. Essentially it is an amount of armor class against spells.
Many spells have spell resistance checks where the caster must overcome the target's spell resistance (see: ).
The calculation is a pretty simple one and works like this:
Caster: 1d20 + Caster Level + Feats + Bonus for Assay Resistance Defender: Spell Resistance amount + penalties to spell resistance
The caster must equal or beat the defender's spell resistance. There are no critical hits and even a roll of 20 may fail.
Spell resistance does not stack, only the highest is applied (if you have SR 25 vs. evil, and SR 20 vs. anything, against an evil caster you only have the higher 25 amount). However, penalties do stack, such as from Breach.
The feats which affect spell resistance checks are:
Effects when using Spell Mantle
Spell resistance is done after any checks for spells such as Spell Mantle, Globe of Invulnerability or Spell Immunity are done, or any items or powers that grant similar effects (such as Shield granting immunity to Magic Missile).
Having a Spell Mantle up therefore will work before Spell Resistance is checked. This is more inline with the standard D&D rules, since immunities to attacks (eg. via Concealment) in melee are done before attack roll checks.
Using the numbers
We can assume casters are in equal level to the defender for all intents and purposes. Therefore, we can take into account a spell like Spell Resistance which gives 12 + 1 / caster level worth of SR.
Therefore, as noted in the notes for Spell Resistance, this applies the bare minimum for a good defensive check against equal level casters - a 12 or more on a d20 is needed for an enemy caster to affect you, although this is lowered to needing an 10 or 8 depending on what feats the caster has (Spell penetration and Greater spell penetration respectively).
Usually monsters have similar levels of spell resistance - 10, 11 or 12 + their level. Higher level monsters may have lower amounts compared to their hit dice, while still having many (such as dragons and daemons).
Countering high spell resistance
These spells lower spell resistance by the amount specified (the effects stack):
- Lesser Spell Breach - 3, 60 seconds
- Greater Spell Breach - 5, 60 seconds
- Mordenkainen's Disjunction - 10, 60 seconds (to all in area including allies)
- Nature's Balance - (Affects enemies in an area, requires a failed will save) 1d4 / 5 caster levels, for 1 rounds/3 caster levels. - NWN Spell was not implemented in NWN2
Also, Assay Resistance means the caster gains +10 in checks specifically against the targeted creature (although no other caster benefits from this bonus).
- SR applied through feats like Diamond Soul or Githyanki SR racial feat cannot be lowered by spell breaches, Mordenkainen's Disjunction or Nature's Balance. The only ways to break through such SR are spell penetration feats or the Assay Resistance spell. Same problem may also occur with targets whose SR is realised as a hide property (Succubbi, for example, whose SR is a property of their "hide" they are technically "wearing" in their "chest" slot).
- Area spells like Cloudkill ignore the caster's spell penetration feats (if any), because any checks are not performed directly between caster<->target, but instead filtered through the AoE object. So if a caster lvl 11 with the Spell Penetration feat were to cast Cloudkill on an enemy with SR 14, the enemy may actually resist the spell, because the Spell Penetration feat is ignored in this case.
- n.b. Spell Resistance granted by equipped items seems to be bugged (v.1.23). This does not apply to a character's Innate or spell-granted SR, but only to equipped items; after a character makes a spell resistance check against a hostile spell, the SR granted by any item will be incorrectly removed and subsequent hostile spell attacks effectively bypass the character's Spell Resistance. (Check the SR value on your Character Sheet to verify, then unequip/re-equip the item to get your SR back.) Opponent monsters might also suffer this dismal drawback - although there are fixes available on the Neverwinter Vault. (Google Searches for further information: bug fix)
- A Warlock's invocations are also apparently affected by SR issues (Spellcasting: Invocations, see also links above). Warlocks use only half their caster level for the purpose of overcoming opponents' Spell Resistance, and may have further problems with Spell Penetration feats.
Because of its real-time nature, Bioware/Obsidian elected to make friendly abilities never use spell resistance, while all hostile abilities do.
To overcome a creature's spell resistance, the caster of the spell or spell-like ability must equal or exceed the creature's spell resistance with a caster level check (1d20 + caster level). (The defender's spell resistance is like an Armor Class against magical attacks.) If the caster fails the check, the spell doesn't affect the creature. The possessor does not have to do anything special to use spell resistance. The creature need not even be aware of the threat for its spell resistance to operate.
Only spells and spell-like abilities are subject to spell resistance. Extraordinary and supernatural abilities (including enhancement bonuses on magic weapons) are not. For example, the fear effect from a rod of lordly might is subject to spell resistance because it is a spell-like effect. The rod's combat bonuses (such as the +2 bonus from the rod's mace form) are not. A creature can have some abilities that are subject to spell resistance and some that are not. For example, an androsphinx's divine spells are subject to spell resistance, but its roar is not. (The roar is a supernatural ability.) A cleric's spells are subject to spell resistance, but his use of positive or negative energy is not. Even some spells ignore spell resistance; see "when spell resistance applies", below.
A creature can voluntarily lower its spell resistance. Doing so is a standard action that does not provoke an attack of opportunity. Once a creature lowers its resistance, it remains down until the creature's next turn. At the beginning of the creature's next turn, the creature's spell resistance automatically returns unless the creature intentionally keeps it down (also a standard action that does not provoke an attack of opportunity).
A creature's spell resistance never interferes with its own spells, items, or abilities.
A creature with spell resistance cannot impart this power to others by touching them or standing in their midst. Only the rarest of creatures and a few magic items have the ability to bestow spell resistance upon another.
Spell resistance does not stack. It overlaps. If a cleric wearing +1 chainmail that grants him spell resistance 15 casts holy aura, which grants spell resistance 25 against evil spells and spells cast by evil creatures, he has spell resistance 25 against the aforementioned spells and spell resistance 15 against other spells and spell-like abilities.
When spell resistance applies
Each spell description includes an entry that indicates whether spell resistance applies to the spell. In general, whether spell resistance applies depends on what the spell does:
Targeted spells: Spell resistance applies if the spell is targeted at the creature. Some individually targeted spells, such as magic missile when cast by a 3rd-level caster, can be directed at several creatures simultaneously. In such cases, a creature's spell resistance applies only to the portion of the spell actually targeted at that creature. If several different resistant creatures are subjected to such a spell, each checks its spell resistance separately.
Area spells: Spell resistance applies if the resistant creature is within the spell's area. It protects the resistant creature without affecting the spell itself.
Effect spells: Most effect spells summon or create something and are not subject to spell resistance. For instance, summon monster I summons a monster that can attack a creature with spell resistance normally. Sometimes, however, spell resistance applies to effect spells, usually to those that act upon a creature more or less directly, such as web.
Spell resistance can protect a creature from a spell that's already been cast. Spell resistance is checked when the creature is first affected by the spell. For example, if an ogre mage flies within 10 feet of a wall of fire, the caster must make a caster level check against the ogre mage's spell resistance of 18. If the caster fails, the wall does not damage the ogre mage.
Spell resistance is checked only once for any particular casting of a spell or use of a spell-like ability. If spell resistance fails the first time, it fails each time the creature encounters that same casting of the spell. Likewise, if the spell resistance succeeds the first time, it always succeeds. For example, a succubus encounters Jozan's blade barrier spell. If the cleric makes a successful roll to overcome the spell resistance of the succubus, the creature takes damage from the spell. If the succubus survives and enters that particular blade barrier a second time, the creature will be damaged again. No second roll is needed. If the creature has voluntarily lowered its spell resistance and is then subjected to a spell, the creature still has a single chance to resist that spell later, when its spell resistance is up.
Spell resistance has no effect unless the energy created or released by the spell actually goes to work on the resistant creature's mind or body. If the spell acts on anything else (the air, the ground, the room's light), and the creature is affected as a consequence, no roll is required. Creatures can be harmed by a spell without being directly affected. For example, a daylight spell harms a dark elf because drow have light blindness. Daylight, however, usually is cast on the area containing the drow, making it bright, not on the drow itself, so the effect is indirect. Spell resistance would only apply if someone tried to cast daylight on an object the drow was holding.
Spell resistance does not apply if an effect fools the creature's senses or reveals something about the creature, such as minor illusion or detect thoughts does.
Magic actually has to be working for spell resistance to apply. Spells that have instantaneous durations but lasting results aren't subject to spell resistance unless the resistant creature is exposed to the spell the instant it is cast. For example, a creature with spell resistance can't undo a wall of stone that has already been cast.
When in doubt about whether a spell's effect is direct or indirect, consider the spell's school:
- Abjuration: The target creature must be harmed, changed, or restricted in some manner for spell resistance to apply. Perception changes, such as nondetection, aren't subject to spell resistance. Abjurations that block or negate attacks are not subject to an attacker's spell resistance -- it is the protected creature that is affected by the spell (becoming immune or resistant to the attack).
- Conjuration: These spells are usually not subject to spell resistance unless the spell conjures some form of energy, such as Melf's acid arrow or power word stun. Spells that summon creatures or produce effects that function like creatures are not subject to spell resistance.
- Divination: These spells do not affect creatures directly and are not subject to spell resistance, even though what they reveal about a creature might be very damaging.
- Enchantment: Since enchantment spells affect creatures' minds, they are typically subject to spell resistance.
- Evocation: If an evocation spell deals damage to the creature, it has a direct effect. If the spell damages something else, it has an indirect effect. For example, a lightning bolt cast at a resistant creature is subject to spell resistance (which would protect only the creature but would not affect the spell itself). If the lightning bolt is cast at a chamber's ceiling, bringing down a rain of debris, it is not subject to spell resistance.
- Illusion: These spells are almost never subject to spell resistance. Illusions that entail a direct attack, such as phantasmal killer or shadow evocation, are exceptions.
- Necromancy: Most of these spells alter the target creature's life force and are subject to spell resistance. Unusual necromancy spells, such as spectral hand, don't affect other creatures directly and are not subject to spell resistance.
- Transmutation: These spells are subject to spell resistance if they transform the target creature. Transmutation spells are not subject to spell resistance if they are targeted on a point in space instead of on a creature. Transmute rock to mud and entangle change a creature's surroundings, not the creature itself, and are not subject to spell resistance. Some transmutations make objects harmful (or more harmful), such as magic stone. Even these spells are not generally subject to spell resistance because they affect the objects, not the creatures against which the objects are used. Spell resistance works against magic stone only if the creature with spell resistance is holding the stones when the cleric casts magic stone on them.
Successful spell resistance
Spell resistance prevents a spell or a spell-like ability from affecting or harming the resistant creature, but it never removes a magical effect from another creature or negates a spell's effect on another creature. Spell resistance prevents a spell from disrupting another spell.
Against an ongoing spell that has already been cast, a failed check against spell resistance allows the resistant creature to ignore any effect the spell might have. The magic continues to affect others normally.