At Petersen Games we love monsters. In the course of doing our first three Sagas for Sandy Petersen's Cthulhu Mythos, namely: Ghoul Island, Yig Snake Granddaddy, and Dark Worlds, our writers and developers invented more than 100 new encounters. While these are custom tailored to those Sagas, being mostly Cthulhu Mythos related, they could fit into any dangerous and epic 5e campaign! Thus we are reproducing every single encounter from these Sagas here for easy reference. Plus, we're throwing in eleven brand new monsters never before seen, just for this book! (from the book introduction)