Fidel, Relentless Avenger

Flying, vigilance, lifelink, haste
Vengeance (Whenever this creature is dealt damage
by a source an opponent controls, it gains a +1/+1
counter. When this creature dies, it deals damage to
each opponent equal to the number of +1/+1 counters
on it.)

If a creature you control would deal vengeance
damage to a player, it also deals that much
damage to each creature that player controls.