Master Strike

Upon reaching 20th level, a rogue becomes incredibly deadly when dealing sneak attack damage. Each time the rogue deals sneak attack damage, she can slay the target. The target receives a Fortitude save to negate the death effect. The DC of this save is equal to 10 + 1/2 the rogue's level + the rogue's Dexterity modifier. Once a creature has been the target of a master strike, regardless of whether or not the save is made, that creature is immune to that rogue's master strike for 24 hours.