Free Will: will offer you a 50% chance to shrug the initial cast of a mysticism spell. It provides no benefits after you have been mysted
Myst Resistance: will reduce the duration of the affect, the shorter the duration the better chance to shrug it.
Amulet of Free Will: Provides both Free Will and Myst Resistance
Executioner's Axe: Provides only Free Will
Khan Helmet: Provides only Free Will
Does free will stack?: No. Free will is a single affect and wearing multiple items with free will does not improve you chance to shrug