The Nightmare Child was a new type of Dalek created by Davros during the first year of the Last Great Time War. Davros saw it as the "perfect Dalek," rising above the flaws of every other Dalek. (PROSE: The Third Wise Man)
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) Cass Fermazzi had an encounter with the Nightmare Child. (PROSE: The Day of the Doctor)
The Nightmare Child quickly came out of control, wanting to consume everything like a hungry child. Davros lured it to the Gates of Elysium where he planned to destroy it. He called the War Doctor there, gloating as he and his command ship was swallowed by the Nightmare Child. The Child itself fell into the Gates, and they closed and disappeared behind it. (PROSE: The Third Wise Man)
Rassilon's plan to return Gallifrey to the universe would have allowed the Nightmare Child to come out of the Time War and into reality, in addition to similar horrors from the conflict. (TV: The End of Time)
Behind the scenes Edit
Coincidently, 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.