The character understands the ebb and flow of the forces of magicka, and can manipulate it to disrupt other mages.
As a reaction to a spell cast, the character may make a -20 Mysticism skill test. If their degrees of success exceed the spell level of the spell being cast, then the effect of the spell is negated. Spell effects or magical abilities which do not have Spell Levels cannot be negated in this way.