Hemnus was a legendary demon created by the Outsider to punish the beings who had "ruined the universe's chances of being a Template:TTparadise", such as the Great Vampires and the Kotturuh.
Because most of these races had already been destroyed by wars or by the Kotturuh, Hemnus instead raged unchecked across the universe, until the Time Lords managed to trap him on a tomb on a distant planet, Valderaaz. River Song and Bernice Summerfield later uncovered this tomb, and, realising what they'd found, resolved to call the Doctor for help. (PROSE:: Expectations)