The only known Tomb Nightmare was created by a student of Necromancer Munne, as the target of the quest The Necromancer's Novice. The creature killed its creator and took up residence in the catacombs. Tomb Nightmare is protected by Flame Geysers, you need to trigger the 3 traps with bone horrors, otherwise you will need to kill your self twice to get there (a devourer triggers the first one), or have a significant amount of health to absorb the damage (upwards of 300).