Black Streams: Cults of Ruin

From publisher blurb:

Gain further insight into the dark powers of the Sunset Isles with Cults of Ruin. Inside this free supplement you'll learn truths about five different foul sects. The devotees of the Black Emperor, the slaves of the Hell Kings, and stranger faiths still are given in this text along with guidelines for using them in your own campaign.

As a bonus, you also get the Azure Inquisitor, a Labyrinth Lord-compatible class dedicated to the rooting out of vile cults and secret cabals.

While written for the Red Tide Campaign Setting and Sandbox Toolkit, this supplement's information is easily reskinned for your own home campaign, with abominable faiths and foul creeds for your heroes to smite with righteous fury.