A place where the whole world takes place in the mind and soul of the person 'entering' it! This would be Sorcerer heaven, designed by some mad Master Sorc who created an object or location based spell. Ideally the experience would be different each time and tailored to the character who goes into the place.
I'm not sure how mechanics could work but I'm sure there's ways to make several scenarios possible. Also, what would be nice is that ANY level would be able to go in, since each nightmare would be unique to the individual.

I would imagine creatures in the area to be sorcerer oriented to some extent, perhaps offshoots of the mad sorcerer. The way experience would manifest
in an entirely illusion based experience would be the concept of dream world becoming real. If the balance of having a world where any level can enter
and hunt within it, then possibly only solo hunting would do it. This way, when the character dies then a rescue is possible where they entered, since they never physically left but are returning to the 'real' world. As for treasure, maybe a demon can port it for you at a hefty fee between worlds? Or none but you get something for it on the outside when you return...