Limit Breaker (3.5e Feat)
From D&D Wiki
 Limit Breaker [General Feat]
This allows the character to further push themselves in life or death situations, allowing them to perform greater feats than even they imagined.
Prerequisite: Diehard, fortitude save +6 (base+Con bonus), 6th level, Con 13
Benefit: You suddenly feel an inner strength replacing the weakness you felt only moments earlier. Your movements grow faster, attacks are stronger, and you feel as though you can fight anew. Once per encounter, when your hit points drop below 0, you can choose to take a fortitude save (DC 15+damage dealt) or become unconscious and stable. Success means you are granted temporary hit points for 1 round per character level equal to your Constitution score. You are treated as not being disabled and able to act normally during those rounds. You also gain a +10 ft bonus to movement, a +2 bonus to Strength, and a +1 morale bonus to attack and will saves. Failure of the save means you can act as though disabled. At the end of every one of your turns, you lose 1 temporary hit point. If you lose all of your temporary hit points, the save can't be attempted again and you fall unconscious and do not automatically become stable (overriding Diehard). Either way, after the duration is up, you become exhausted and take an additional 1d6 Str ability damage.
Normal: With Diehard, you can act as disabled after falling below 0 hit points or become unconscious and stable.
Special: This feat is useful for those who are in the thick of combat, and a high Constitution score is necessary to benefit best from this feat. Just as a notice, the player can choose to fail the Fortitude save to act as though disabled.