Cults of Prax, by Steve Perrin and Greg Stafford, is a book of deities and, since the RuneQuest system and the world of Glorantha are both intimately connected to the many religions contained within, it is one of the game’s most important books. Prax is both a melting pot and the site of a clash of cultures on an epic scale, so the gods detailed in the book are a cross-section of deities found throughout Glorantha. Several pantheons – those of the native nomad tribes, the invading Lunar Empire, the neighboring Kingdom of Sartar, the city of Pavis and more – all compete for influence in the region. If you play RuneQuest, you will know the names of gods like Orlanth, Humakt, Storm Bull, the Seven Mothers, Lhankor Mhy and Yelmalio very, very well.

Unlike D&D, which features endless deities but few doctrines that matter in terms of the game’s mechanics, the cults are RuneQuest’s foundation. Membership in one of more cults determines not just a character’s culture and beliefs, but many also function as a more organic sort of character class.

In D&D, a book like this would only be of use to Clerics and Paladins, and probably only in expanding the available spells. In RuneQuest, religion is roleplaying – Cults of Prax is a handbook for how all characters (player and non-player) act, both with members of their own cults and outsiders. Learning these customs and how to navigate them is the soul of the game.

I suspect the centering of the interaction of beliefs on the level of social groups in RuneQuest is a clear antecedent of Planescape and its factions. It is testament to the monumental nature of the book that, even though it was originally released in 1979, it has yet to meet its match in depth and clarity. It is a foundational RPG text (and not just because it might be the first splatbook – correct me if I am wrong in the comments).