Vulnerability

(Spell Compendium, page 232, DnD 3.5)

Transmutation

Level: ; Assassin 4; Cleric 5; Sorcerer/Wizard 5
Components: V, S
Casting Time: 1 standard action
Range: Touch
Target: Creature touched
Duration: 1 round/level
Saving Throw: Will negates
Spell Resistance: Yes

You invoke the power of this spell and a purple mist swirls around your opponent, weakening its mystic protections.

This spell lowers the subject's damage reduction by 5 (to a minimum of 5). For instance, if you successfully cast vulnerability on a dragon with damage reduction 10/magic, its damage reduction becomes 5/magic.

For every four caster levels beyond 9th, the subject's damage reduction lowers by an additional 5: a reduction of 10 at caster level 15th and a reduction of 15 at caster level 19th.