A Manifestation of Oblivion is some kind of apex predator dream creature that can be manifested from the Great Dream.

Despite the name, manifestations of oblivion are not related at all to the concept of forgetting. The name might come from the fact that they are known to eat every kind of dreamstuff around them, and if none is available, they will turn on regular people and start attacking them instead, eating their memories.

Apparently they can be summoned by using the true name of some kind of eldritch deity, as the Bowler-Hatted Man did during the Halloween Session.