Concept: “Worship him, fight him, have him as a dungeon boss, it matters not. He is eternal and omniscient.”
Content: An eldritch abomination from elsewhere
Writing: Includes stats and a table of strange effects that occur in the creature’s presence
Art/design: Blackletter font and line art on a white ground
Usability: Easy to use but probably pretty lethal

Concept: “It is difficult for the lacklustre mind of your average dungeon diver to conceive such a creature, building on images one has seen before in the real world.”
Content: An extradimensional cosmic horror that feasts on sanity
Writing: Provides (anti-)lore and the sanity mechanics by which it interacts with PCs
Art/design: Text arrayed around a central image; typefaces and orientation delineates verbal sections
Usability: Incorporates an ad-hoc sanity point value, but it’s intuitive and easy to manage 

Concept: “Through many alchemical experiments you have mastered not only your wit and intellect but also, well, actually that’s it. But still you’re smart!”
Content: A brainiac character class
Writing: Standard class profile with origins and special features
Art/design: Borgy but readable graphic design with an appropriately bizarre illustration
Usability: You don’t have to be a big thinker to play one