Coup de grâce is a special attack that is automatically made when attacking a helpless creature with four or fewer hit dice. For the purpose of a coup de grâce, "helpless" means paralyzed or sleeping. This special attack can be made with either a melee weapon or a ranged weapon if the attacker is within 30 feet of the target (the same range used for sneak attacks). When a coup de grâce is made, the attack is automatically successful, and the victim dies from the attack. (There is no specific damage dealt; the victim is simply killed.) While a coup de grâce cannot be applied to a player character, it can still instantly kill associates, summons, familiars, and animal companions of that player.
Pen and paper
Coup de grâce as implemented in Neverwinter Nights differs significantly from the pen and paper (PnP) version. In PnP, a single coup de grâce replaces all attacks in the round (rather than just one attack) and provokes an attack of opportunity from threatening foes. The PnP coup de grâce is not limited by the hit dice of the victim; instead it allows a saving throw to avoid death, with a DC based on the damage dealt. This save is made more difficult by the attack automatically being a critical hit (and a sneak attack, if applicable). If the creature is immune to critical hits, the PnP coup de grâce is avoided.
One other notable difference is that a PnP coup de grâce can only be made with a ranged weapon if the attacker is adjacent to the victim, while Neverwinter Nights allows the attacker to be up to 30 feet away.