Blood Frenzied (3.5e Trait)
From D&D Wiki
When your life is in danger, the feel of your blood being shed drives you into a berserk rage.
Benefit: When your hit points are reduced to 20% or less you gain a +1 bonus to attack rolls and damage rolls.
Drawback: When your hit points are restored to greater than 20%, you take a -1 penalty to attack rolls and damage rolls for 1 hour.
Roleplaying Ideas: Characters with this trait may be played in a variety of ways. They could have an innate rage that triggers at the sight of taste of their own blood, or it may simply be a manic desire to survive that drives them to fight when it counts most.