Werewolves in canid form are supernaturally strong and tough, and since the weakening of Avacynian magic, few protection spells have been able to harm them or keep them at bay. But werewolves have a weakness: pure silver that has been ritually blessed by a powerful cleric of Avacyn can cause them great agony. According to alchemists, silver's purity of material readily absorbs the divine magic. Arrowheads, spearpoints, and other weapons made from blessed silver can be powerful instruments for fighting werewolves.

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PLANESWALKER'S GUIDE TO INNISTRAD: KESSIG AND WEREWOLVES