Devastating Critical

 
Prerequisite(s): 21+ level, 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.
 
Source: Epic Level Handbook, Draconomicon