Blood Frenzied (3.5e Trait)
From D&D Wiki
 Blood Frenzied
When your life is in danger, the feel of your blood being shed drives you into a berserk rage.
Benefit: When your health is down to 20% or less, you lose control, adding +3 to all attack and damage rolls.
Drawback: While under the effects of this trait, you cannot perform any action that does not involve attacking or moving closer to an enemy. When the encounter is over or you are healed above 20% health, you fall unconscious for 1d6 rounds.
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.