I've always enjoyed a great eldritch horror game: Call of Cthulhu or more recently, Trail of Cthulhu. And I really enjoy the Cypher System, the core rules used in Numenera and The Strange. Now, with the publication of the Cypher System Rulebook, I can mix my shuggoths and my GM Intrusions.
So, how does horror work in the Cypher System? First, I should explain a little about how the Cypher System Rulebook works. It's a menu of rules a GM (and players) can chose from to create their own game. Besides horror, there are rules for science fiction, fantasy, modern, and superheros. Within each genre, you get more choices. Lots of optional rules to tailor the game to just the right kind of horror.
The rulebook has a big bestiary of creatures to choose from and the horror section lists the ones that best fit a horror campaign, including Deep Ones and Mi-Go. It also includes 3 horror artifacts, so if you want more, you're going to have to make some yourself.
The horror rules are all optional rules. Madness will be familiar to Cthulhu players. Cypher System madness grew out of the madness rules found in In Strange Aeons: Lovecraftian Numenera. In this case madness puts a hurt on your Intellect pool and can even change your descriptor. I do something similar in The Sun Below: Sleeping Lady, which has a seriously eldrtich horror vibe.
Shock is another optional rule, more for short term effects like losing control of your character and running away or sitting down to sob rather than fight. Trail of Cthulhu players will find it very similar to Stability losses. Different mechanics, but going for the same effect.
Combine both Madness and Shock if you want a game like Trail of Cthulhu, or just Madness if you want a Call of Cthulhuish experience.
I've saved the most interesting for last. Horror Mode. Horror mode is a great way to ramp up tension, as the range for a GM Intrusion (the dice fail kind, not the 2xp kind) goes up to a 1-2 on a d20 roll. Then 1-3. And so on. You can get to 1-10 fairly quickly. Each GM Intrusion brings new horror AND widens the GM Intrusion range by one.
After the horror mode is over, you reset the GM Intrusion range to 1. Until the next time...
Which has me thinking of a Cypher Cthulhu adventure. I'm sure my players would love it!