Sanity

Lol of Cthulhu.

Sanity, in roleplaying games, is a measure of how mentally stable a character is. In traditional roleplaying games, such as AD&D, a character's mental state was entirely up to how the character's player wished to portray them; however, Call of Cthulhu, based on the Cthulhu Mythos by writer H.P. Lovecraft, features the progression to insanity as a major feature of the setting and gameplay, and introduced the Sanity attribute to track the character's descent into madness. Players would "roll for SAN loss" every time they experienced something unpleasant or unsettling, like seeing a corpse, or reading the Necronomicon; if they failed to roll under their current Sanity, they lose some random number of SAN points, and lose a smaller number (or none at all) if they succeed, eventually becoming a gibbering and/or villainous NPC when their Sanity reaches zero. The concept proved extremely popular, and has since been modified and incorporated into a wide variety of different RPGs, either as a core part of the system or as an optional rule for gamers running horror-based campaigns.

One of the most notable updates of the Sanity mechanic (which was something like mental hit points in older games) has been carried out by games such as Unknown Armies. UA sanity works in such a way that, while it can make you more used to seeing horrible things (becoming "hardened" in respect to some horror, like eviscerated bodies), it also makes you colder, insensitive and so on (more likely to carry out violent deeds), while in Call of Cthulhu you just kept losing your sanity points until you went crazy and had to be institutionalized.