Requiem for a God — Monte Cook

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.