This description is from the Chaosium Website used by g2ch with the permission of Chaosium Inc.

Here are those things which can ?eternal lie?, and which have lived beyond human understanding for strange aeons. The Cthulhu mythos is comprised of a complex and broad group of sometimes-contradictory entities, powers, and concepts that encompass the secrets of time, space, and the universe. Fundamental truths of the universe are so alien and horrifying to humankind that mere exposure to them might result in madness or suicide. While humanity might crave comfort and truth, only one or the other is possible.

This tome contains entries for more than 380 different creatures and beings that are part of the Cthulhu mythos, presented for use with the call of Cthulhu roleplaying game. Entries are organized into the following types:

Outer gods rule the universe. All races and lesser deities of the mythos acknowledge the outer gods, and may worship them. Except for Nyarlathotep, these gods have little to do with humanity.

Elder gods are neutral to or possibly rivals to the outer gods. Though vast and of awesome power, they do not seem to be as dangerous to humanity as the outer gods.

Great old ones are not omnipotent, but nonetheless are godlike and terrible in human eyes. Humans are likely to worship great old ones, who are comparatively near at hand.

Great ones are the gods of earth?s dreamlands. Do not confuse them with the great old ones. They are the weakest of all the deity types, and a wise mortal can surpass them in might.

Avatars are variant manifestations of an outer god, great old one, elder god, or great one. Usually less powerful than the gods they represent, many are more horrible to witness.

Some entries introduce unique entities, defying classification. Races independent of and serving no particular deity are included, though individuals may worship, cooperate, or co-conspire with such beings. Fabulous creatures are well represented, drawn from legend and lore. Animals & non-mythos monsters complete the presentation, chosen from those most likely to be encountered, or the most common or popular.

The Malleus Monstrorum is a whopping 296 page soft cover, perfect bound book with black and white illustrations and a full index.