Déagol was a friend of Sméagol. He discovered the One Ring on a fishing trip after it had lain dormant for thousands of years. Sméagol, who was with him at the time, then demanded the ring be given to him as a present, since it happened to be his birthday. Déagol refused, so Sméagol killed him and took it for himself, claiming it as a "birthday present".
Encyclopedia entry originally written by ~Mirimë~