Quagmire in The Elder Scrolls Online.

"Every few minutes, there was a flash of lightning and reality shifted, always to something more horrible and horrifying."
Seif-ij Hidja[src]

Quagmire, the realm of Nightmares, is the plane of Oblivion that is associated with the Daedric Prince Vaermina. Every time lightning strikes, reality shifts to another horrifying vision, as described by Morian Zenas. It could be anything, from a dark castle, a den of terrifying beasts, a moonlit swamp, or even a casket in which one sees oneself buried.[1]

In 4E 201, during the events of Skyrim, Vaermina's Skull of Corruption has been instrumental for her to worm her way into the sleeping minds of the residents of Dawnstar. The nightmares they experience very likely occur in Quagmire.[2]