Bloodrage is a Warrior ability.
(Passive) Infuriated by wounds, the hero deals +1 damage for every 2 health they are missing.
Half the total Injury, round up.
Note that temporary health gained, such as from Aid, provides extra temporary hp but does not reduce Injury. This means, for instance, that with Bloodrage, if you take 4 damage then get 4 temporary hp, you still have 4 injury and do +2 damage. But if it happens in the other order, if you get 4 temporary hp first and then take 4 damage, you have 0 injury and do +0 damage.