Planescape - Doors to the Unknown

From the web:

Doors to the Unknown is a collection of four Planescape adventures that can be played separately or as a mini-campaign. When four doors appear in the Cage, the barmies crawl out of the shadows, and the heroes get drawn into events that could have consequences for the entire multiverse. Each door leads to a different plane and a different deadly challenge for the player characters. Together they offer a way to stop an ancient menace before it strikes again.