Lowest accessible board of reality in "JAGS: Wonderland", or the realm of Chokmah in "KULT" - they, or rather their variations as presented by me, are pretty similar.
JAGS:Wonderland seemed really interesting to me, but it looked like a challenge to play. How did that go?
The strangest place my players visited so far was my own creation, the Cosmic Library. It was built a long time back by an interstellar race of giant spiders who had conquered much of the galaxy, and is basically a library the occupies an entire planet. The star system where this planet exists has started to go red giant, so the planet itself is wrapped in a time distortion field where everything happens very quickly, so it's been about five minutes till destruction for the last hundred thousand years of use (you get there through wormholes, since the planet is already gone in the current timeline). Yes, the idea for a planetary-scale library on the verge of destruction was stolen from Dr. Who.
The race that made the library has since devolved into their current forms, which are (mostly) mindless spiders of all varieties; modern spiders are their descendants. The various races of extant giant spiders are the remnants of their old castes. To get around the library, you need to hire one of the Librarians, who are a race of four-armed tattooed blue giants able to (usually) safely guide visitors through the wilderness of The Stacks. In return for this service, they charge one of your fondest memories. While my players visited, they found a reading room with "mind-books" on crystal cylinders that gave them some random skills (some useless, some not).
The system is Lamentations of the Flame Princess, and yes, this is a fantasy setting.