The Conclave
From UnderlightWiki
The Conclave is accessed through a hidden portal in Harrow Glades.
Much of the plane has not been 'uncovered' and so unusable portals exist there.
The Conclave also features unique 'rooms within rooms', areas in rooms that look ordinary, though dreamers and talismans are invisible across the boundary. A dreamer appears to be using a portal when they step into the area, and standing in the area results in a different output when using Know, however the transition between is seamless, and the feature would be easily missed by a dreamer on their own.