Magic resistance is a natural immunity that confers a percentage chance of any spell absolutely failing when used against the creature. The percentage indicates the effectiveness of magic resistance against an 11th level caster. Casters are 5% less effective against magic resistance per level below 11th; and 5% more effective per level above 11th.
For example, a creature with a 35% magic resistance would have that resistance improved 15% if attacked by an 8th level caster; conversely, an 18th level caster would be high enough level so as to eliminate all resistance possessed by the defending creature.
If the roll is successful for the creature, all effects of the spell are nullified. If the creature's resistance fails, the creature is still entitled to any saving throws the spell would normally allow.
Magic resistance also affects the potential for monsters to break through magically-formed barriers, such as hold portal or forbiddance. If met with such a barrier, the creature rolls against its resistance (adjusted by the level of the barrier's caster) — success indicates the barrier is dispelled, not only for the resistant creature but for all others.
Magic Items, such as scrolls, wands, rods and staves, add +6 to the effective level of the user with respect to magic resistance (but not to the item's power). Thus, a 6th level mage using a wand would be considered 12th level when using that wand against a creature with magic resistance.
Creatures with the Resistance
Monsters known to possess magic resistance include: