Challengers of Olympus
A character’s Sanity Rating is equal to his willpower.
Anytime the character is exposed to a Mythos creature for the first time must roll a sanity check (sanity rating +d10) modified by the creature’s fear rating. The first time a character succeeds at this roll he gains 1 rank in Mythos.
When a character reads a mythos book roll intelligence (not doubled). For every success the character gains 1 rank in Mythos and must make a Sanity check (modified by the ½ the character’s mythos skill). If the character fails, make a sanity check at -5.
When a character casts a spell (Will + Mythos) make a sanity check modified by the spells power level. If the spell fails roll on the spell failure chart. If the spell fails the sanity check is made at -5.
When a character encounters a strange/creepy situation roll a sanity check. Include any appropriate modifiers.