Trefusis

Miss Trefusis was head prison guard of Broadfell Prison, which held Lucy Saxon, as well as a member of a 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, killing her, 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.