The Immortals were a group of Time Lords stuck as statues in perpetual regeneration on the side of the Rassilon's Tomb. (PROSE: The Five Doctors [+]Loading...["The Five Doctors (novelisation)"]) This was achieved by wearing the Ring of Rassilon after winning the Game of Rassilon. (TV: The Five Doctors [+]Loading...["The Five Doctors (TV story)"], PROSE: The Five Doctors [+]Loading...["The Five Doctors (novelisation)"])
By the time of Borusa's Game of Rassilon, there were three prior Immortals, with Borusa becoming the fourth. (TV: The Five Doctors [+]Loading...["The Five Doctors (TV story)"], PROSE: The Five Doctors [+]Loading...["The Five Doctors (novelisation)"], The Four Faces of Immortality [+]Loading...["The Four Faces of Immortality (short story)"])