Aura of Divine Retribution

An Aura Divine Protection and Vengeance radiates from the caster protecting him and his allies and rebuking those who would seek to do them harm.  Until the spell ends the aura moves with caster and is centered on the caster.  For the duration of this spell the caster and all friendly characters  within 10 feet of the caster receive a +2 bonus to AC.   As a reaction once per turn, when any friendly character affected by this spell was successfully damaged by melee attack from an opposing character, the caster may deal that attacker 1d6+Charisma modifier Radiant damage.   This spell ends when the caster dealt damage from this spell a number of times equal to his Spell Casting Ability Modifier.   

This aura increases to 30 ft at 18th level.