Crash Course: Arkham Mythos

The Call of Cthulu Card game use insanity as a method of character control. Certain characters have this terror icon, that is you total up and see who has the most. The loser has to turn one of their characters face-down (considered insane), and this character cannot have a terror icon themselves (they are immune to being insane). There are a total of like four different icons, terror is the first you resolve.

At the start of the turn you turn a face-down character face up, but exhausted so that it is still inactive till the next turn.

The board game uses it similar, just as you have HP total, you have a sanity total, if that ever reaches 0 you basically lose a turn. There are many different ways to lose sanity, like casting spells, losing certain event rolls.

Hope this helps.

In the CoC RPG, it's like mental Hit Points. Some monsters and events are more mind-bendingly awful and terrifying than others. When you encounter them, you have to make a Sanity Check (roll less than or equal to your Sanity on a d100). If you fail, you lose some Sanity points (1 to 4 for "minor" things like stumbling upon a putrefied corpse, much more for Elder Gods like Azathoth or C'thulhu.). If you lose all of your Sanity, you go insane. If you lose a certain percentage of your Sanity all at once, you get a Phobia or other psychological ailment. As you may expect, triggering your phobia may cause you to... lose more Sanity.