The Ring of Rassilon (or the Ring of Immortality) was worn by Rassilon, who slept eternally in the Tomb of Rassilon while his mind lived on in the Matrix. The fate of anyone who wore the Ring of Rassilon was told in the rhyme, written in Old High Gallifreyan.

To lose is to win
And he who wins shall lose

When the Ring was worn it granted the wearer immortality and imprisoned the wearer in the stone of Rassilon's embossing, watching over Rassilon forever. (TV: The Five Doctors)

The ring could also be used to open up a hidden door in the Tomb. Rassilon possessed Leela as she wore the ring, leading her, Romana II, and K9 Mark 2 into the Matrix. (AUDIO: Zagreus)
