Call of Cthulhu Sanity Loss per Encounter

Sanity loss in Call of Cthulhu

Sandy Petersen designed Call of Cthulhu, and Chaosium put it out in 1981, fitting H. P. Lovecraft’s mythos onto the Basic Roleplaying d100 system. Every Investigator starts with SAN equal to POW × 5, and that ceiling drops to 99 − Cthulhu Mythos. Encounters write the Sanity loss as X / Y. A mutilated corpse might run 0/1D3; a Mythos monster 1/1D10; laying eyes on Cthulhu himself 1D10/1D100. You take the left number when the SAN check succeeds and the right one when it fails.

Lose 5 or more SAN in a single roll and temporary insanity kicks in. Drop a fifth of your current SAN over the course of a day and you face indefinite insanity. The Cthulhu Mythos skill works against you here: the more it climbs, the lower it pushes your maximum SAN, so the longer an Investigator survives and learns, the less sanity is ever left to recover.

Applications

Keepers lean on a calculator like this to roll encounter losses mid-session, working out the success and failure branches before the player even makes the d100 SAN check. Players, meanwhile, watch how close they are to the next bout of madness, or to slipping under the 80% mark that flags fragile sanity.

FAQ

How does the X/Y notation work? You roll d100 against your current SAN. On a success you lose X, on a failure you lose Y. Either side can be a flat number or a dice expression.

When does temporary insanity hit? Any time one Sanity check drains 5 or more SAN. The Investigator then suffers a bout that lasts 1D10 rounds, stretching into hours or days depending on what caused it.

Does learning Mythos really cost SAN? It does. Each point of Cthulhu Mythos knocks one point off your maximum possible SAN, the rules’ way of showing forbidden knowledge wearing the mind down for good.