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.

There is not, and there will never be a stat block for these things because they can not be hurt, and they either instakill dreamstuff or erase memories with no save roll. They are more of a moving hazard than a mob, and that's why they are in the thing namespace instead of the mob one.