Nightmare Child

The Nightmare Child was something born during the Time War. Davros' command ship flew into the "jaws" of this creature during the first year of the war. Despite the war being Time locked, Dalek Caan managed to enter the war with an emergency temporal shift. (DW: Evolution of the Daleks) He arrived in the middle of the battle and saved Davros. (DW: The Stolen Earth)

The Tenth Doctor stated, "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 in December 2009 would have led to the Nightmare Child, in addition to similar-such horrors like the Skaro Degradations, 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. However, the Doctor managed to prevent this from happening by severing the link that the Time Lords had to the rest of the universe, thus returning them to the inevitable oblivion of the Last Great Time War. (DW: The End of Time)

It has not yet been precisely explained what the "Nightmare Child" actually was. It is unknown if it was an object, a creature, or a cosmic phenomenon, and it is unknown if it was associated with a side in the war or was an unaligned force of mayhem.