This is Cthulhu Dark, the 2017 print-on-demand version that is a 180+ page hardcover book. I point this out because the original Cthulhu Dark that I own was published as a tri-fold pamphlet back in 2012. That pamphlet contained all the rules you need to play. The big ol’ hardcover reprints them pretty much word for word. It does not expand them. They amount to a two-page spread. The rules are obviously very light and attempt to distill the sort of occult investigation-style game of a Call of Cthulhu or similar to the barest elements. It does this very well.

Basically, the intent is a failure-free system. Die rolls (on a D6) indicate the degree of success from 1 (barely) to 6 (brilliantly and possibly beyond human ken). This is balanced by the way the game is organized around the idea of investigators being doomed and the successful solving of mysteries likely consuming them. Whenever an investigator sees something disturbing, they roll Insight (which starts at a value of 1). Roll higher than their current value, they are unsettled and add 1 to their value. Reach 6 in this way, they perceive the truth of the universe and leave conventional existence behind in whatever manner the player decides. Elegant!

Then what’s in the rest of the book? Several excellent scenarios and a lot of support material the will help GMs write their own scenarios or convert scenarios from other systems. I must admit, I have been tempted (though I think the Dark system would consume player characters too fast to make playing longform Call of Cthulhu campaigns like Masks of Nyarlathotep feasible).