The Mouth of the Underworld was possibly first seen in the episode Once more the Nightmare Factory . The creature was seen as a large, blue and green, swirling, ragged toothed mouth. It is usually depicted as having a monster-like body as well. It is un known what Argost has hiding behind that wall in his home. It also might be possible that Argost uses this creature to lure away or send his enemys to the underworld.
The Mouth of the Underworld has been said to be related to the leviathan's mouth and cerberus's mouths.