SRD:Vorpal Strike

Prerequisites
Str 25, Wis 25, Improved Critical (unarmed strike), Improved Unarmed Strike, Keen Strike, Stunning Fist, ki strike (adamantine).

Benefit
The character’s unarmed strike is considered to be a slashing vorpal weapon. (At the character’s option, any unarmed strike can do bludgeoning damage instead, but it loses the vorpal quality.) This ability doesn’t stack with similar abilities.