The Hybrid

The Prophecy of the Hybrid, (PROSE: Dalek Combat Training Manual) otherwise shortened to simply 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) According to the Dalek Combat Training Manual, the text of the legend dated back to the Cloister Wars, (PROSE: Dalek Combat Training Manual) while another account claimed 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)

The Dalek Combat Manual, even despite being part of a confidential chapter intended only for section leaders, blanked out what the text of the legend actually entailed. (PROSE: Dalek Combat Training Manual) During the Last Great Time War, an alternate Davros separately proposed to both the Eleventh General and a Supreme Dalek that he could create the Hybrid to win the conflict. When he attempted to create the creature based on a mixture of his and the Doctor's genetics, the retro-regeneration the Doctor was affected by caused the specimen to collapse. (AUDIO: A Genius for War)

On the last day of the conflict, 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) 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

 * The Hybrid prophecy is played out throughout Heaven Sent and Hell Bent. Then the Doctor and Clara, of two warrior races, stand in the ruins of Gallifrey at the end of the story, we also see the Doctor unravel the Web of Time saving Clara from death and the Doctor also kills a billion billion hearts to heal his own when the Doctor kept burning himself up to restore his own body in the Dial.
 * 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.