From publisher's site:
The cults dedicated to Glorantha's deities are among the oldest and most powerful institutions on the face of Glorantha. Contained in this book are the secret lore and powers known only to the most ardent worshippers of the gods, whether they are the faithful warriors battling in the Storm King's image for the cause of good, or vicious, uncivilised trolls emulating the evils of Zorak Zoran in the name of personal power.
In the Second Age, the Godswar is a distant time legend and myth, but the divine warriors that shed their blood so long ago now thrive on the respect and dedication of mortal followers. Cults of Glorantha: Volume I opens the way for players to join their cults of choice and rise through the ranks from mere believers to Runelords and Runepriests, going to war and adventuring across the world in the name of their god - calling down holy power and honouring the pantheon with grand deeds.