This system allows you to introduce an element of dark horror into your game. In campaigns using these rules, characters gain a new attribute called Sanity.
This statistic functions like an ability score in some ways, but it has its own unique mechanics that represent the character’s descent from a stable and healthy mental state into confusion, dementia, and mental instability. As a character encounters monsters, witnesses horrible acts, masters forbidden knowledge, or cast spells, his Sanity score, and his corresponding ability to function as a normal member of his race, deteriorates. This gradual descent is balanced in part by the powers that characters gain each time they overcome a horrific foe or grow in skill and expertise, but even as those characters grow in power, they know or fear that an even greater peril lays ahead —
the threat of becoming permanently insane.