Sirens of Time

The Sirens of Time were a species that fed insatiably upon the energy released by temporal distortions caused by changes in history. They could not change history themselves, only manipulate others into doing so. Those who answered the call of the Sirens twice were eternally enslaved to them.

One avatar of the Sirens appeared three times in the Doctor's timeline. It first appeared to the Doctor as Helen, a crew member of a British merchant ship sunk by a German U-boat in 1915, and manipulated him into preventing the sinking of the RMS Lusitania. It then appeared to the Doctor as Ellie, a waitress on board the starship Edifice, and manipulated him into freeing the Temperon and the Knights of Velyshaa. It then appeared to the Doctor as Elenya, the only survivor of a spacecraft crash, and manipulating him into saving Sancroff.

The result of these changes was that the Knights of Velyshaa were able to found a Second Empire, discover time travel and invade Gallifrey. The avatar of the Sirens then appeared to the three Doctors (who were brought to Gallifrey by the Temporon) as Knight Commander Lyena, who tried to manipulate them into undoing the changes they made to history. Doing so would have permanently enslaved them into creating more temporal distortions until the Universe itself would collapse. The Doctors instead freed the Temporon, who sacrificed its freedom to contain the Sirens in eternal struggle. (BFA: The Sirens of Time)