For Halloween, I tried out the horror mode rule from the Cypher System. I ran a horror Numenera game down at my Friendly Local Game Shop (Modern Boardshop), where I reskinned an adventure for horror. I used The Nightmare Switch, from the original Numenera Kickstarter, which worked well.
Horror mode changes the way the players' die rolls can create GM Intrusions. Instead of only on a 1, you start off at a 1 or 2, and keep adding to the threat range for the GM Intrusion. Once we got it up to 8, and with six players rolling dice, we had a huge number or GM Intrusions.
The Cypher System book has all the details, but the idea is that GM Intrusions bring on the horror. Once the PCs beat a local horror, or successfully flee from it, you set it back to normal (only a 1 gives a free GM Intrusion).
To help create the right mood, I took a huge red d20, and slapped it down on the table with the 2 up. Then I explained the rule. Every time I upped the chance to get a GM Intrusion, I picked it up and slapped it down with the new number up. I may have chuckled evilly.
So what about when you want the horror to show up? Easy. Just use a normal GM Intrusion, hand out the 2 xp. And add one to the horror die. Slap!
How did it work? It worked great. The players focused on their die rolls, constantly glancing at the big red die in, well, horror.
I'm coming to appreciate when a game designer like Monte Cook uses a core component (GM Intrusions) of their existing system in a new way. This reminds me of why I like how 13th Age designers have come up with new ways to use the escalation die and icon relationship roles. I find this so much better than the old way of slapping on a new mechanic to the side of an existing game. Eventually the system is defined by a collection of non-related mechanics that have been shoved into the game until I find it unplayable.
I can totally see adding horror mode to any cypher system game where I have a horror component. In fact, it would work great in The Sun Below: Sleeping Lady, which has a strong eldritch horror vibe.
I think I know what my next Sleeping Lady Extra will be...