Cthulhu Dark is a tabletop roleplaying game of cosmic horror, in the style of H.P. Lovecraft.
It's about the things we fear, amplified until they are unbearable. It's about stories that genuinely creep you out, not well-worn tropes and creatures you've seen hundreds of times before.
And it's about bleak horror, in which humans are powerless when confronted by hyperintelligent alien horrors. You can't beat them. You can't fight them. You can only watch, run, hide and fear.
If you love Cthulhu games already, then this is Lovecraftian horror at its most intense. If you're new to them, then Cthulhu Dark is a great place to start. You'll enter a world of stories that are both terrifying and terribly human.
The rules are incredibly short: they're tightly focussed on horror and they drive the game.
Have a look at these samples of the book: the rules, the start of the Keeper's section and one of the settings, London 1851.