My idea for this has always been:
- Sanity (takes damage like HP) = 10 + ( Will + Presence + Logic ) x 2
- Only has one defense – Your Resolve score
- When reduced to Zero Sanity, you don’t die, you receive a flaw (those in the Core Rules are a good starting point) in the form of a mental illness
- Add a “cooldown” mechanic where, if you go for X amount of time without taking Sanity damage, one of your flaws suffered from Sanity damage is removed.