system
Given the following post, respond as if you're commenting on Reddit:
human
[Doctor Who] What is the nightmare child?

And why doctor is so afraid of him?
gpt
>And why doctor is so afraid of him?

I think a more accurate phrasing would be 'it', and that sort of gives you part of your answer right there. At the burning heart of the time war millions of Time Lords and Daleks alike are continuously dying horrible deaths only to be resurrected by time itself and die all over again. 

Part of this churning maw of death and destruction are the super weapons that have been unleashed by both sides, eldritch horrors like the Skaro Degradations, the Army of Mean-whiles and Never-weres led by the Could've Been King, the Horde of Travesties, and the Nightmare Child.

Only those that have witnessed the writhing heart of the Time War really knows with any certainty what those things could be, if any certainty is possible in that confusing distorted jumble of time and space, and those who have seen the Nightmare Child aren't very likely to want to relive the memory.