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. (TV: Hell Bent) It was the first prophecy that was made by the Matrix, but it was ignored at the time given that it was forecasted to occur billions of years in the future. (PROSE: A Brief History of Time Lords) Nonetheless, the Time Lords were certain it would come eventually. (TV: Heaven Sent)

On the last day of the Last Great Time War, the Visionary had predicted Gallifrey would fall. (TV: The End of Time) When the planet survived the Time War, (TV: The Day of the Doctor) Rassilon began to fear that the Hybrid would bring about the prophesied fall. Convinced by that the Twelfth Doctor knew about the creature, (PROSE: A Brief History of Time Lords) he trapped and imprisoned the Doctor in his confession dial 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.
 * The events of The Timeless Children fit multiple aspects of the Hybrid. For example, 's fusing with the Cyberium made him a hybrid between two warrior races: the Time Lords and the Cybermen. Additionally, the CyberMasters were Cyber-Warriors converted from dead Time Lords, thereby making them a hybrid of the Time Lords and the Cybermen. Furthermore, the Master stood over the ruins of Gallifrey after learning the story of the Timeless Child and murdering the Time Lords in his rage; thereby "breaking a billion hearts to heal [his] own". However, no official source, in-universe or not, has made any correlation between the events of The Timeless Children and the Hybrid.