Blood Vengeance is a bless effect that triggers whenever a unit takes damage, forcing the attacker to take a magic resistance check or take an equal amount of damage. This is very similar to damage reversal, with the only difference being that a failed resistance check does not negate the original damage.
Blood vengeance is not prevented by Ethereal, Luck,Twist Fate,Mossbody or Mistform. If the attacker fails the magic resistance they take AN damage equal to the damage dealt to the defender.