Syriath

According to Jack Harkness, Syriath was an entity from "the oldest universe of all". "She" was trapped underneath the Cardiff Space-Time Rift before time itself.

Due to an alignment of events — including the presence of the psychic Mrs Wintergreen in the House of the Dead, built on a ley line — she found a way to escape in the early 2010. If she had reached the world, she would have ripped it apart, using the dead as a weapon. Since she came from before time, she could bend it; she sent people visions of dead people. Mrs Wintergreen summoned her and, later, was possessed by her. Among the others, Syriath had evoked the dead Ianto Jones to use against Jack Harkness. The echo of Ianto used a giant battery of stored rift energy to seal the rift forever and wipe out Syriath. (AUDIO: The House of the Dead)