The Hybrid

The Hybrid was a Gallifreyan legend that said all Matrix prophecies predicted that a hybrid creature, thought to be crossbred from two warrior races, would stand over the ruins of Gallifrey and unravel the Web of Time, breaking a billion billion hearts to heal its own. Rassilon searched for information about the Hybrid. (TV: Hell Bent)

Behind the scenes

 * Although Hell Bent, among other series 9 episodes, offered multiple possibilities as to the identity/nature of the Hybrid, showrunner Steven Moffat, in an interview for Doctor Who Magazine #504, confirmed that he intended that the Hybrid was the pairing of the Doctor and Clara. He retroactively linked this fact to the reason why the Doctor's TARDIS exhibited some hostility towards Clara in series 7.
 * In series 12, opening two-parter Spyfall had reveal that he had destroyed Gallifrey and left it in ruins. In the second part of the two-part finale, The Timeless Children, has the Doctor actually stand in the ruins of Gallifrey; technically bringing the prophecy to pass.