Cthulhu sleeps in the corpse city of R'lyeh. Through unblinking eyes he watches mountains grow and empires rise and fall. He lays quiet, unmoving. Now he stirs. He senses that the stars are once again converging into their proper formations. His worshippers have plotted for the day when their mighty lord once again walks the earth. That day has come! Shadows of Yog-Sothoth presents a grand, seven part campaign, revealing details of the Silver Twilight, an international organization dedicated to forcing the premature rise of R'lyeh and the release of the dread Cthulhu. (from the back cover)

This is a reprinted and expanded version of the very first Call of Cthulhu supplement from 1982. It preserves the original artwork and seven scenarios as well as expands upon handouts, statistics, and descriptions. It also includes two bonus scenarios that are made for stand alone play (The People of the Monolith and The Warren). Overall, this set of linked scenarios is intended for experienced investigators (after all, they will be trying to stop the return of Cthulhu!). The People of the Monolith is the only scenario in the book that is made for beginning investigators.

Shadows of Yog-Sothoth is a 150 page soft cover, perfect bound book with black and white illustrations and maps throughout. There are also another 23 pages of player handout materials at the back of the book.