From publisher blurb:
A succubus with an unquenchable lust for power is turning the Cage into a ripping madhouse.
And the Lady of Pain ain't pleased. . . .
Something's foul in Sigil, berk. A mad slasher prowls the streets, leaving a trail of bloody clues in his wake. Leatherheads boldly court the Lady of Pain, longing for the sharp touch of her shadow. Even the silent dabus're acting barmy, a sure sign that the Lady herself is uneasy. Are these strange events tied to a madhouse full of spell-touched sods ready to slip the bonds of mortality? A curious body'd better step carefully. After all, the dead-book's full of fools who wished for power?or got in the way of another berk's wish.
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 wails to be discovered in the lunatic asylum called Harbinger House, and only the player characters can shape its power-for good or ill.
The Planescape Campaign Setting boxed set is required to run this adventure. The Planescape Monstrous Compendium Appendix, In the Cage: A Guide to sigil, and A Player's Primer to the Outlands are recommended to enhance your enjoyment of this adventure.