The first name that comes to mind when most people think of Cthulhu is HP Lovecraft. If you're a gamer, probably the second name you think of is Sandy Petersen. He's the go-to authority for Cthulhu mythos in the gaming world and he's bringing that knowledge to one of the most popular RPG systems out there with his Cthulhu Mythos book for the Pathfinder RPG. It's up on Kickstarter now.
So, what can you expect in this sanity-stretching book? Lots, actually. There's over 100 monsters to send at your players, including many (over half) that have never had rules in Pathfinder before. Even those that have been seen before are getting a "Mythos Makeover." That is, they've had their rules updated to fit in with the others. There's also creatures that are sort of "Mythos Adjacent." They're not really part of the mythos, but probably could be. There's also new plot hooks and campaign jumping-on points to get your players headed in the right direction. Plus, Sandy helps out if you've never run an horror-style RPG before. If you're going to find out how to do it, might as well find out from one of the best.
The campaign's just underway and is already more than 3x funded and working through stretch goals. It's set to last another 29 days.