Jagged Tooth

(Spell Compendium, page 126, DnD 3.5)

Transmutation

Level: ; Druid 3; Ranger 2
Components: V, S
Casting Time: 1 standard action
Range: Close (25 ft. + 5 ft./2 levels)
Target: One natural slashing or piercing weapon of target creature
Duration: 10 minutes/level
Saving Throw: Will negates (harmless)
Spell Resistance: Yes (harmless)

You feel your own teeth sharpen momentarily as you complete this spell. As the magical energy surrounds your intended target, it flexes its muscles in appreciation of its weapon's deadlier capabilities.

This spell doubles the critical threat range of one natural weapon that deals either slashing or piercing damage. Multiple effects that increase a weapon's threat range don't stack.