The mad, banished elder gods who roamed the heavens before the pantheon — imprisoned in the Nothing and driven insane.
The Feral Gods are, per Juice Box, "the gods from before" — immortal beasts who roamed the heavens before the current pantheon banished them into the Nothing and created the world. Their endless banishment has driven them mad. Opening the Gate of the Feral Gods tears a hole into the Nothing, and when it closes one of these feral gods comes through onto the *opening* side, bringing devastation.
Mordecai has long warned Carl about the word "feral": "Stay away from anything marked that. It's always bad news." The title artifact, the boss goose Denise, and the tomb's whole cast of misplaced, Nothing-warped creatures all orbit these ancient horrors.
Ancient immortal god-beasts, unseen; imagined as maddened primeval divinities warped by aeons in the void.
The maddened elder gods the Gate would unleash — the cosmic threat lurking behind the whole fifth floor.