Helpless

A helpless character is paralyzed, held, bound, sleeping, unconscious, or otherwise completely at an opponent’s mercy. A helpless target is treated as having a Dexterity of 0 (–5 modifier). Attacks against a helpless target get a +5 bonus with Advantage. A helpless target may be sneak attacked.

As a full-round action, a character may use a weapon to deliver a coup de grace to a helpless foe. The attacker automatically hits and scores a critical hit. Sneak attack damage is added to the coup de grace. If the defender survives, they must make a Constitution save (DC 10 + damage dealt) or die. Delivering a coup de grace provokes attacks of opportunity.

Creatures that are immune to critical hits do not take critical damage, nor do they need to make Constitution saves to avoid being killed by a coup de grace.