The Pirates Of Atlantis
The character has the ability to work himself into a bloodthirsty rage while in combat. When he becomes berserk, his Strength increases and he feels less pain, but he also loses the ability to distinguish friend from foe; he simply attacks the person nearest to him with the most powerful attack he can muster, and he never takes defensive actions.
In game terms, the character gains an Aid STR 3d6 (which fades at the normal rate of 5 Character Points per Turn) and Physical Damage Reduction, Resistant, 25%, STUN Only (which lasts only as long as the Aid effect lasts). However, activating the power makes him Berserk, and he cannot recover from being Berserk until all STR gained from the Aid fades.