| Enraged Retaliation is quite similar to the utility power for the Pit Fighter fighter paragon path, which is a daily ability that triggers off taking damage from a melee attack and grants the user a +2 to attack and damage rolls against the enemy that damaged him until the end if his next turn.
If Enraged Retaliation is pretty much at-will, I would limit it to a small damage bonus.
If it's effectively an encounter ability, e.g. it triggers the first time you get bloodied in an encounter, I could allow a small attack bonus as well.
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