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

I have known about its existence for a long time but I've never actually bothered to find out what it is and for some reason I kind of want to know now.
gpt
If you're asking that question, then you couldn't possibly fathom the true horror of the Nightmare Child. The existential angst of all those who might have existed, who could still exist but don't. The unborn of all space and time. People think the "Clockroaches" that straighten out paradoxes are the worst consequence of muddling about with the timeline. Those people haven't seen the Nightmare Child.