A door sized for Medium creatures appears on a flat, solid surface. It leads to a demiplane that is an empty room sized 30ft x 30ft x 30ft. The room is made of wood or stone. When the spell ends, the door disappears, and anything inside the demiplane is trapped there.
When you cast the spell, you can make a new demiplane, connect to an old one you created, or connect to one another creature made if you know the nature and contents of the demiplane.