ToME: the Tales of Maj'Eyal

Had a level 35 nightmare horror spawn in Lake of Nur on a low level (13) necromancer. It was able to kill me in two rounds even with pretty high con and blurred mortality (lv3) on. (effectively had 400 hp) I understand OOD enemies are a part of the game but I was dead on two occasions before it even came into line of sight and it's ability to follow anywhere on the level, move through walls, and move quickly makes it an insurmountable challenge at anything but the high end of the Lake of Nur cap.

you go there too early, and Nightmare Horror is actually one of the player killer.
you should go back here later like level 18 something

Lake of Nur can spawn stuff like that...The best thing you can do is run away, if you manage.