He belonged to the race of the Black Númenóreans and briefly appeared in person when he haggled with the Army of the West in front of the Black Gate (Morannon in Elvish), trying to convince Aragorn and Gandalf to give up and let Sauron win the war for Middle-earth.
Unwillingly he agreed, and as the thing got into the bed, it turned into an elvish woman, who was clad in silk and who was the most beautiful woman he had ever seen.