Minor spoiler warning! The following section contains minor spoilers. You may want to refrain from reading it.
Death, or the Reaper, is a character found on a boat in the Underworld. He cannot be attacked, and any attempt to do so will teleport the attacker away from him. He possesses Death's Scythe, one of the most powerful weapons in the game. It is possible to steal his scythe when in the form of a Nymph, but he will always have it when you return since the Underworld is "outside of time."