Coup de Grace
Sometimes a creature has the chance to attack a foe who is completely defenseless. Doing so isn’t chivalrous, but it is viciously effective. This action is known as a coup de grace.
Perform a Coup de Grace
Action: Standard action.
Attack Helpless Target: The creature uses one of its attack powers against an adjacent target that is helpless. If the attack hits, it automatically scores a critical hit against the target.
Slaying the Target Outright: If the critical hit deals damage greater than or equal to the target’s bloodied value, the target dies.
The most common way for a target to be helpless is to be unconscious; thus, the coup de grace action is typically used against sleeping foes.
Published in Player's Handbook, page(s) 288, Rules Compendium, page(s) 241.