An event book by Monte Cook.
From the back of the book:
A God Dies...
... and while the faithful mourn, others make their move.
Malhavoc Press proudly presents the first d20 event book, a "how-to" guide for players and DMs. Requiem for a God supposes that a major event has taken place in your campaign: the death of an immortal being.
This book has all you need to introduce the demise of a divine power into your game. DM notes cover the death's repercussions on the campaign, and new prestige classes tap into the departed god's power. Short tie-in adventures feature power-mad villains seeking to hoard their new might while cosmic forces move to realign the balance.
Characters can wield artifacts made of god-relics and spells fuelled by lingering divine essence. Meanwhile, dark creatures born of the god's passing challenge adventurers of all levels.
Kill a god in your game. Then watch what happens.