Astral Aperture

Camouflage. While inactive Door Mouth is invisible except for its mouth.

Dungeon Mouth. Its Door Mouth's mouth looks like a door. If a creature would open the door it lead to an extradimensional space and looks like a passage down into a dungeon of the DM's choosing. If a creature would enter the character starts to feel oppressed by the damp feeling inside. The stairs descend 40 feet before coming to the entrance of a small dungeon. The dungeon is empty. 2 rounds after the door has been opened it starts to close, after 5 rounds the door is locked and the characters cannot get out. The dungeon has enough air for 1 hour divided amongst the creatures. tiny creature count as 1/2 and large creatures count as 2. 10 minutes after the door is closed the floor becomes sticky and tries to pull the players down making it difficult terrain. Once the door is closed and characters are in it the door disappears into the astral plane

Achilles heel. the Door Mouth is immune to all damage except for one patch of floor that looks like skin in the middle of the dungeon. on that spot the Door Mouth is vulnerable to all damage except bludgeoning.

Dead Teleportation. once the Door Mouth is killed the door open up to a random location of the DM's choosing

Since the Door Mouth is technically in the Astral plane it cannot be attacked from the outside because it isn't there.

Create Traps(3/character inside).  As an action the Door Mouth can create any trap of the DM's choice. The trap has to be made in a space where the character cannot see it being made. The trap can be in a place the party has already traversed.