True Death Attack

If an assassin studies his victim for 3 rounds and then makes a sneak attack with a melee weapon that successfully deals damage, the sneak attack has the additional effect of killing the target if the victim of such an attack fails a Fortitude save (DC 10 + the assassin’s class level + the assassin’s Int modifier). If the victim’s saving throw succeeds, the attack is just a normal sneak attack. Once the assassin has completed the 3 rounds of study, he must make the death attack within the next 3 rounds.