The number one most famous mechanic in the Call of Cthulhu game is Sanity. There's a lot of details to it but ultimately what it means that you can see or hear or learn something so awful it hurts your mind. Now when I was first designing Call of Cthulhu, I wanted to make it as much like the stories as possible (that's always my first step in a game - to put theme first). And in Lovecraft's tales, the heroes would faint dead away, panic, go into hysterics, become lunatics, and so forth, so I wanted this in the game. It seemed like a good fit for a horror game. So I assigned the players a good amount of Sanity (typically 50+ points), and each monster could inflict a potential Sanity loss. In my naivete, I thought that this would just be an additional threat for a monster. You know - "A Dark Young can hit you with tentacles, cast spells, and also cost you up to 1d10 Sanity. Boo!" 1/3