Six centuries ago, events surrounding the Great Upheaval imposed an uneasy peace in the City of Doors. Since that time, the 15 factions have quietly conducted their philosophical battle for the hearts and minds of all Cagers - and always stopped short of all-out war. But now tensions are flaring, tempers are rising, and the Cage is about to explode into conflict! Faction War is a 128-page adventure that tracks the war from its nefarious origins through its bitter battles to its ultimate resolution and beyond - and puts the future of Sigil in the hands of the player characters. This book reveals secrets, unravels mysteries, brings long-simmering plots to a head, and marks the beginning of a new era for the City of Doors. It also features: • New details on Sigil's wards, personalities, and politics. • Adventures that draw the player characters into the heart of the struggle and toward the secrets surrounding the war's beginning and ultimate end. • Extensive notes on the aftermath of the conflict. • A detailed Timeline of the war, including additional events that the DM may use expand the adventure. Faction War builds on story elements first explored in the PLANESCAPE accessories In the Cage, Factol's Manifesto, and Uncaged: Faces of Sigil. Familiarity with those products is recommended but not required.