SRD:Devastating Critical

Prerequisites
Str 25, Cleave, Great Cleave, Improved Critical (chosen weapon), Overwhelming Critical (chosen weapon), Power Attack, Weapon Focus (chosen weapon).

Benefit
Whenever the character scores a critical hit with the chosen weapon, the target must make a Fortitude save (DC 10 + 1/2 character level + character’s Strength modifier) or die instantly. (Creatures immune to critical hits can’t be affected by this feat.)

Special
A character can gain this feat multiple times. Its effects do not stack. Each time a character takes the feat, it applies to a different type of weapon.