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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 of the Sirens appeared three times in the Doctor's timeline. It first appeared to the Fifth Doctor as Helen, a crew member of a British merchant ship sunk by the U-20 in 1915, and manipulated him into preventing the sinking of the RMS Lusitania. This led to humans never going into space due to being devastated by disease due to someone from the ship killing the chemist Alexander Fleming. It next appeared to the Sixth Doctor as Ellie, a waitress on board the starship Edifice, and manipulated him into freeing the Temperon and the Knights of Velyshaa. It finally appeared to the Seventh Doctor as Elenya, the only survivor of a spacecraft crash, and manipulated him into saving Sancroff, who helped re-inspire the Knights of Velyshaa.
The result of these changes was that the Knights of Velyshaa were able to found a new empire, discover time travel and invade the planet Gallifrey. The Siren appeared to the three Doctors (who were brought to Gallifrey by the Temperon) as Knight Commander Lyena, who tried to manipulate them into un-doing the changes they had made to history. Doing so would have permanently enslaved them into creating more temporal distortions until the universe itself would have collapsed. The Doctors instead freed the Temperon, who sacrificed its freedom to contain the Sirens in eternal struggle. (AUDIO: The Sirens of Time)