The Lost Valley
Location: Outlands / Hinterlands
In the Hinterlands, just past the gate town of Xaos, lies what is commonly called the Lost Valley. This is because things lost (both literally and figuratively) often end up here. A mystical city lies in the valley, although very few people are let into it—and the chant goes that that advanced magic powers much of the city. What is known is that the souls that live here are the last remnants of some vanished civilisation. Apparently the Lost Valley was once only a place where things that were misplaced occasionally appeared (a lot of luggage, socks, and hair). That was true until the mystical city in the prime started to fall. The mystical and psionic effects maintained by the king faltered as the wizard-king slowly lost his sanity.
Taking the idea of the legendary “lost realm” a bit literally, these clueless berks actually managed to send an entire city into the planes. Here, their belief in finding his lost mind changed the nature of the realm. Now lost marbles, lost battles, and even entire sects following lost causes are just as likely to appear as stuff literally lost. Of course most lost stuff doesn’t appear here anyway, it’s just misplaced behind furniture, but every so often a portal appears which carries the lost items here.
Source: Joshua Jarvis