Nightmare Child

Considered an abomination by those who saw it, the Nightmare Child was the first and only specimen of a new type of Dalek, created by Davros during the first year of the Last Great Time War. Inspired by how the Doctor disliked the Time Lords but nevertheless helped them, Davros saw it as the "perfect Dalek," rising above the flaws of every other Dalek, as it was programmed to not revere the Dalek species as perfect so it could evolve and improve. Thus, upon being created, it rejected its body out of the scorn and horror it held towards it. It then hacked into Dalek systems, consuming regular Daleks in its path. (PROSE: The Third Wise Man)

History
The Nightmare Child quickly became out of control, wanting to consume everything like a hungry child. Davros planned to destroy it by luring it to the Gates of Elysium. He called the War Doctor there, gloating as he and his command ship were swallowed by the Nightmare Child, despite the Doctor's efforts to save Davros. (PROSE: The Third Wise Man) Another account claimed the Eighth Doctor, the incarnation of the Doctor before the War Doctor, had been present instead. (PROSE: The Whoniverse)

The Time Lord strike-force that came with the Doctor, meanwhile, was thrown into chaos, finding themselves in the midst of the insanity that was the battle between Daleks and the Child. Their TARDIS scanners could not make sense of the Child other than some recognisable aspects made from Dalek tech. Appearing like a dark blizzard or swarm, the Child had made itself millions of small pieces, all moving together in a shape that looked like a jellyfish or a shoal of fish, to attack and consume thousands of Daleks. As Davros's command ship pushed it into the gates, the child sent out shining tendrils, trying to feed, escape, or multiply, but the command ship fired to hold it back. (PROSE: The Third Wise Man)

However, after escaping New York, Dalek Caan fell into the Time War and saved Davros at the cost of his own sanity. As Dalek Caan recounted, "[he] flew into the wild, and the fire; [he] danced and died a thousand times." (TV: The Stolen Earth) Afterward, the Child and the command ship fell into the Gates, and they closed and disappeared behind it. (PROSE: The Third Wise Man) However, the Child survived. (TV: The End of Time)

By the end of the Time War, Earth was duplicated thousands of times over to be used as bullets to fire at the Nightmare Child. (PROSE: Doctor Who and the Time War) Cass Fermazzi had an encounter with the Nightmare Child. The War Doctor fought to prevent the rise of the Nightmare Child (PROSE: The Day of the Doctor) and made arrangements for it "to never arise and [to] forever be aware of its non-existence". (PROSE: Twice Upon a Time)

Rassilon's plan to return Gallifrey to the universe would have allowed the Nightmare Child and other horrors to come out of the Time War and into reality. (TV: The End of Time)

Legacy
When reliving a memory from his eighth incarnation, the Tenth Doctor recalled "laugh[ing] in the face of the Nightmare Child". (COMIC: The Forgotten) The Testimony Foundation had footage of the Doctor trying to defeat the Nightmare Child. (PROSE: Twice Upon a Time)

Behind the scenes

 * According to The Writer's Tale, it was referred to as "the Dalek Emperor's Nightmare Child" in Russell T Davies' early script draft.


 * Coincidentally, the Nightmare Child backstory from The Third Wise Man is similar to the one given by John Freeman for "The New Daleks" redesign Raymond Cusick did for the DWMS Tenth Anniversary Special: describing it as the ultimate Dalek and ultimate killing machine coming from plans drawn by Davros.


 * The writer of The Third Wise Man, Dave Rudden, stated in a Reddit AMA that he felt he did not fully explain the Nightmare Child, instead only showing a fragment of its newborn form. To Rudden, after going through the gates, the seven deaths of Davros, and more, the Child had much growing to do, especially so if it was to reach the power it had in Russell T Davies' Doctor Who and the Time War.