Fearful Ends is a rules-light, story-centric roleplaying system for horror themed games. It features nihilistic stories about characters discovering impossible horrors. It aims to allow players to roleplay characters that experience mental or emotional collapse in a safe play environment that neither stigmatizes nor sensationalizes mental illness.
We've been playing and refining Paul's system for horror games for a bunch of years, and it really runs well. I've even GM'd a session of it myself with a custom scenario very successfully, without needing to be a deep expert in this flavor of RPG. Easy to pick up and learn, and we always have a great time with it.