Vulnerability

(Draconomicon, 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

This spell lowers the subject's damage reduction by 5 points (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 11th, the subject's damage reduction lowers by an additional 5 points: 10 at 15th and 15 at 19th.