CALL OF CTHULHU MALLEUS MONSTRORUM KEEPER DECKA range of monsters with game characteristics and evocative artwork. Keeper Decks are designed to provide in game resources, handy references, and information. Use the Malleus Monstrorum Deck when you need to reference a monster profile and when designing scenarios and encounters. These decks provide new layers to plots, enhance the story, and provide quick access to an array of fiendish foes.