The floor is covered in shadows.
|Hidden in the shadows is a trap door covering a physical pit. When activated, it becomes a gate to a small demiplane.
A DC 14 Wisdom (Perception) check reveals a 10’x10′ rectangular outline of seams in the floor in one of the shadows in front of the door. If the target has his own light source, this check is at advantage. This is a trap door.
When the trap door is stepped on, it opens into a 10′ pit. However, at the same time, a gateway is activated that opens a 30′ cube demiplane. Then the gateway closes and the pit is just a 10′ pit again.
Creatures in the demiplane are trapped in that demiplane and must be retrieved by means of planar travel.
An aura of conjuration can be detected on the trap door with detect magic or a successful DC 22 Intelligence (Arcana) check.
Dispel magic (DC 18) or a DC 24 Intelligence (Arcana) check dispels the portal effect.
The trap door can also be secured with a DC 16 Dexterity check with thieves’ tools.