Planescape - Harbinger House

From the web:

Harbinger House is a PLANESCAPE adventure for four to six characters of 4th to 7th levels. From Sigil to the Outlands and back again, the heroes must piece together a puzzle that could shake the Cage to its foundations. A dark secret of the multiverse waits to be discovered in the lunatic asylum called Harbinger House, and only the player characters can shape its power - for good or ill.