The Faction War is an adventure for Advanced Dungeons & Dragons set in Sigil and launched for the Planescape campaign setting. It is one of several mega-adventures that TSR and Wizards of the Coast used towards Advancing the Storyline, such as how Die, Vecna, Die! was used to cover the transition from 2nd Edition to 3rd edition.
Fueled by his legendary arrogance, Duke Rowan Darkwood, Factol of the Fated, pursues and recovers an ancient Macguffin that nobody ever noticed before this module's plot needed it: an ebony gem, containing the soul of a legendary wizard who wrought a spell intended to seize control of Sigil from the Lady of Pain and render it unto him instead. With this in hand, he sets out to dominate Sigil by manipulating the Factions into open war - at which he succeeds.
The players take up the role of individuals struggling to survive in the resultant chaos, and to figure out what's really going on. No small task, especially as the adventure's climax involves the factions going to open warfare and Sigil being invaded by armies of Tanar'ri and Baatezu hoping to seize advantage of the chaos.
Eventually, the madness is stopped and the ebony gem is lost once more. The Factions make a tentative peace, but their power is shattered -especially when the Lady announces that all of the Factions are now unwelcome, and they must disassemble or die. Some Factions formally renounce their power and maintain their roles, such as the Bleak Cabal, Dustmen and Society of Sensation. Others quit Sigil and retreat to extraplanar locales -- the Fated to Ysgard and the Athar to the Outlands. And three Factions even cease to exist entirely; the Believers of the Source and the Sign of One fuse together to become the new Mind's Eye philosophy, whilst the Mercykillers split up and revert to being the Sons of Mercy and the Sodkillers again.
Needless to say, this adventure was a huge source of skub in its day. Planescape's always been a divisive setting (is the Great Wheel deep and mature, or pretentious and pompous? Is the Blood War the best thing since sliced meatbread or overhyped and boring?) but this adventure took that to new heights. You had people who liked the end of the Factions, people who hated it, people who wished certain different fates for certain Factions, people who didn't like how it kicked off, people who hated the time travel aspect...