You are particularly resistant to Sorcery, and any spell cast at you has an extra chance of failure. Whenever a Sorcery spell is successfully cast against you, or when you come into contact with an exisiting sorcery effect, make a Magic roll against the opposing sorcerer's Magic. If you make it, the spell fails.
This is similiar to, but far from identical to DamageImmunity: Sorcery. You can elect to forego this protection if you wish. You can use this to resist any sorcery effect, including Blast and Summoning.