Nightmare Child

The Nightmare Child was born during the Last Great Time War. Davros' command ship flew into the "jaws" of this being during the first year of the war at the Gates of Elysium. (DW: The Stolen Earth)

The Tenth Doctor said "I saw Arcadia destroyed. I laughed in the face of the Nightmare Child," after reliving a memory from his eighth incarnation. (IDW: The Forgotten)

Rassilon's plan to return Gallifrey to the universe would have led to the Nightmare Child, in addition to similar horrors like the Skaro Degradations, what remained of the Dalek Empire, the Army of Meanwhiles and Neverweres led by the Could've Been King, and the Horde of Travesties, being pulled out of the Time War and into reality. The Tenth Doctor severed the link created by the Time Lords and prevented them from escaping the oblivion of the Last Great Time War. (DW: The End of Time)

Behind the scenes

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