Planar Bestiary
When is a door not a door? One answer is when it's the shapechanged mouth of an extradimensional creature known as a trapdoor lurker.
The only parts of a trapdoor lurker people in Material Plane worlds see are the creature's lips, usually shapechanged to resemble a door, a hatch, or a trapdoor in the floor. The rest of the creature is a limited living dimension, mostly stomach. There is no "outside" to that stomach, at least not one that can be observed in any known dimension, including from the Ethereal.
Even when the lurker's "door" is opened, the likeness to a hallway visible beyond remains strong. So much so that many explorers simply walk in. That's their last mistake. The door slams shut and retracts into the same dimension as the stomach, cutting off the swallowed explorer from their allies and previous plane.
"Where'd that door come from? It's not on the map. We'd better check it out."
Motive: Hungers for flesh
Environment: Basements, dungeons, ruins across the multiverse
Health: 18
Damage Inflicted: Moderate wound (4 points)
Movement: Immediate; immediate when climbing
Modifications: Speed defense as level 2 due to low mobility; disguise (as a normal door) as level 6
Combat: A glance through the lurker's door-like façade reveals a corridor leading onward. A successful level 6 perception task reveals that the perspective in the corridor is oddly "flat." The rich odor of rotting meat is also faintly apparent. But that's it. It's not an illusion; the opening is real (albeit, a dimensional portal).
If an observer decides not to proceed through the door, a tongue curls out to lash them, inflicting damage and grabbing the victim until they escape with a Might-based task as their action. All a grabbed victim's physical tasks are hindered. If the lurker begins its next turn still grabbing the target, it pulls the target into the "corridor" as its action, even as it changes shape to reveal a stomach-like space. The door and its frame also transform, revealing a horrific interdimensional maw.
A creature that enters or starts their turn in the lurker's stomach (either because they walked in, or because they were pulled in by the tongue) is digested each round, automatically taking 4 points of damage from acid. The victim undergoing digestion is held in place and their physical tasks are hindered, but they can attempt a Might-based task to escape from the stomach back through the gaping portal mouth. The lurker regains 1 point of health each time a victim being digested takes damage from acid.
Escape attempts become more difficult when the lurker uses an action to fully retract its mouth, like a turtle pulling its head into its shell, closing the portal between its interior and the dimension where it was hunting. The lurker can remain so "submerged" and untethered to any other dimension for couple of rounds or until it takes 6 points of damage from one attack from inside itself, at which point the "door" reappears in the same location it retracted from, and victims being digested can attempt to escape again. If killed, the lurker's mouth also reappears, and victims can wriggle free.
Interaction: About as intelligent as deep-sea squid, lurkers are single.minded, rarely letting prey go.
Use: A newly finished structure has more doors than the builders put in.
Loot: The lurker's stomach contains undigested oddments, sometimes including coins equivalent to an expensive item (a couple of hundred gold coins) and perhaps a sturdy magic cypher such as the metallic hilt of a monoblade.
The lurker's stomach can hold several human-sized victims at a time.
GM Intrusion: When the lurker re-anchors itself, the escaping character (or characters) discovers the doorway leads to a different location, or perhaps even a different world of the Material Plane.