nightmare
A bad dream. Essentially when a character is dreaming of a bad situation in their sleep, such as facing something that they feared.
Note that this tag is only for literal nightmare, not a figurative nightmare (such as creepy or disturbing imagery). As such, avoid using this tag in horror-themed imagery unless it depicts someone suffering from a nightmare.
For a particularly disturbing image, use Danbooru pool #339 ("Nightmare Fuel") instead.