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)
References edit
The Daleks Regenerate
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 revealed the Daleks would never allow anything to be half-Dalek. (TV: Heaven Sent)
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-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)

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)

When the Master killed everyone on Gallifrey, several elements of the prophecy were fulfilled: The Master merged with the Cyberium and thereby became a hybrid creature; stood over the ruins of Gallifrey; hacked into the Matrix and thereby unravelled the Web of Time; and slaughtered the Time Lords after he became distraught at learning the truth of their origins, thereby breaking a billion hearts to heal his own. (TV: The Timeless Children)