Nightmare Child was a phenomenon or entity that was extant during the first year of the Time War. Davros' command ship flew into the "jaws" of the Child. 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 Nightmare Child could be a telepathic creature (the telepathic pitcher plant see Bliss Star Trek: Voyager) that feeds on starships and their crews, using a neurogenic field to telepathically induce illusions in the crew's minds that make the creature appear to be what they want most, deceiving the crew into bringing their ship to it and entering its maw, passing into its digestion chamber.
Rassilon's plan to return Gallifrey in 2010 would lead to the Nightmare Child in addition to similar such horrors such as the Skaro Degradations and the Hordes of Travesty being pulled out of the Time War and in to this reality. However, The Doctor managed to prevent this from happening by severing the link the Time Lords had to the rest of the universe thus returning them to the oblivion of the Last Great Time War. (DW: The End of Time)
- This seems to indicate the Child might actually be a physical entity
The Tenth Doctor states "I saw Arcadia destroyed. I laughed in the face of the Nightmare Child." after reliving a memory from his Eighth Incarnation. He makes no reference as to what the Nightmare Child may be. (IDW: The Forgotten)
- It has not yet been precisely explained what the "Nightmare Child" actually was.
Since the Nightmare Child is stated to have originally killed Davros, it might be assumed to be a Time Lord or creation of the Time Lords.