Trefusis

Miss Trefusis was head prison guard of Broadfell Prison, which held Lucy Saxon, as well as a member of a sinister cult that worshiped Harold Saxon.

Biography
Trefusis knew about and was sent to collect his ring. (TV: Last of the Time Lords) The Governor, several guards and she resurrected the Master. The Master took her life energy as well as that of several prison staff. (TV: The End of Time)

Behind the scenes

 * The identity of the woman seen collecting the ring at the end of TV: Last of the Time Lords was a matter of much speculation until it was revealed to be Trefusis.