Extreme pain drives you berserk.
Prerequisites: Con 20, Toughness.
Benefit: If you survive 50 points of damage from a single attack and make your Fortitude save (see Massive Damage, on your next action you gain +4 to Strength, +4 to Constitution, and −2 to Armor Class. These adjustments last until the end of the combat, at which time you become fatigued.
Source: Savage Species, page 36. Removed the last Benefits sentence and hyperlinked the fatigued condition.