You can make an extremely deadly riposte if you parry your opponent's attack with exceptional skill.

Prerequisite(s): 9th-level duelist

Benefit: Once per day when you parry an attack and beat the attacker's roll by 10 or more, if you elect to make a riposte following the parry, you can choose before making the attack roll to make the attack into a deadly riposte. If the attack successfully damages your opponent, they must make a Fortitude save (DC 10 + your duelist level + your Intelligence modifier) or die. If your attack misses or deals no damage, this ability is wasted with no effect.