Spellguard:

Once every 8 seconds, marks either an enemy or an allied target within a 12-metre radius for 6 seconds. If a marked enemy is hit with a magic ability, the target takes additional Value unavailable = [(20%)Spell Power] magic damage twice. If a marked ally is targeted with a healing or shielding ability, they receive additional Value unavailable = [(20%)Heal and Shield Power] healing or shield twice.