Devastating Critical( Draconomicon, p. 68)
Choose one type of melee weapon, such as a claw or bite. With that weapon, you are capable of killing any creature with a single strike.
Whenever you score a critical hit with the weapon you have chosen, the target must make a Fortitude save (DC 10 + 1/2 your HD + your character level + your Str modifier) or die instantly. (Creatures immune to critical hits can't be affected by this feat.)
You can gain this feat multiple times. Its effects do not stack. Each time you take the feat, it applies to a new weapon.