The Hybrid

File:The Hybrid Confession - Heaven Sent - Doctor Who - BBC

The Twelfth Doctor confesses about his knowledge of the Hybrid. (TV: Heaven Sent)

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 Missy 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)

References

The Daleks receive the Doctor's regeneration energy from Colony Sarff on Skaro. (TV: The Witch's Familiar)

It was popularly supposed that the two warrior races would be the Time Lords and the Daleks. (TV: Hell Bent) Davros attempted to fulfil the prophecy by giving some of his Daleks regeneration energy taken from the Twelfth Doctor, transforming them into Time Lord hybrids. (TV: The Witch's Familiar) However, the Doctor later believed that the Daleks would never allow anything to be half-Dalek. (TV: Heaven Sent) Davros was left to imagine the hybrid creature that could have been made if he and the Doctor worked together. However, now that he had access to regeneration energy, Davros believed he had great possibilities for future experiments. (PROSE: Secrets of the Dalek Laboratory)

The Twelfth Doctor claimed at various points to possess special knowledge about the Hybrid, going so far as to one point stating that the Hybrid was "me". (TV: Heaven Sent, Hell Bent) It was suggested that knowledge of the Hybrid prophecies from the Cloisters originally drove the Doctor from Gallifrey, and Ashildr proposed that the Doctor himself was a half-human half-Time Lord hybrid. (TV: Hell Bent) However, the author of the Time Lord Academy textbook on Gallifreyan history believed that the Doctor was bluffing when he said that he was the Hybrid and went onto further theorise that the Doctor had no idea what the Hybrid was, merely using the Time Lords' fear of it against them as revenge for killing Clara Oswald. (PROSE: A Brief History of Time Lords)

Other possible candidates for the hybrid included the part-human part-Mire Ashildr, (TV: The Girl Who Died) the simultaneously-human-and-Zygon Osgood, (TV: The Zygon Invasion), The part-organic part-sleep dust Sandmen, (TV: Sleep No More), the "part-human part-wolf" Rose Tyler, the post-metacrisis Doctor Donna, and the half-Time Lord River Song. However, none of these quite fit the prophecy. (PROSE: A Brief History of Time Lords)

In the Cloisters at the end of the universe, Ashildr posited to the Twelfth Doctor that the Hybrid could be interpreted as referring to more than one being: specifically, the pair of the Doctor and his companion Clara Oswald. Just prior to losing his memories of Clara, the Doctor indicated that his reckless actions in trying to prevent her death made him the Hybrid. (TV: Hell Bent) A Time Lord author agreed with this theory, noting it was the only one that truly fit the prophecy. (PROSE: A Brief History of Time Lords)

In 2002, Sam Jones told Griffin about a Gallifreyan prophecy concerning a hybrid. She said the Doctor was a hybrid, his father a Time Lord and his mother a human, and that he was destined to unite the two races and "bring good old human niceness into their alien society". Though she thought she was bluffing to stall Griffin, the Eighth Doctor later asked her who had told her about the prophecy. (PROSE: Unnatural History)

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, the Spy Master'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.