Coup de Grace
As a full action, you can deliver a special attack called a coup de grace to an adjacent helpless opponent. You automatically hit and score a critical hit. If the target survives the damage, he must succeed at a Fortitude saving throw (DC = 10 + your level or CR) or die. However, if the target is immune to critical hits, the coup de grace does not deal critical damage or effects, nor does it force the target to succeed at a saving throw or die.
Source
Core Rulebook pg. 248
