Willpower is a measure of a character’s ability to concentrate, channel his force of will in a particular direction, or harden his psyche to possible mental shock. It is also a measure of personal resolve. The skill is used in all manner of situations where mental resilience is required, and this includes resisting magic. Although not a measure of sanity it can be used to endure traumatic events that would shake even the sanest, stable mind. Willpower is the mental counterpart to Endurance.
Again, like Endurance and Evade, Willpower is most often used in Opposed Tests. When used as a Standard test, a Critical Willpower roll indicates that the character has hardened his mind and spirit to the extent that no further attempts to influence him, or shake his resolve, will work. In the case of resisting magic, a Critical Success means that no further mentally afflicting spells cast by the opponent have any effect on the character for the remainder of that encounter.