When a barbarian enters a Rage — which he may do whenever he has taken damage from a perceived or actual foe — for the remainder of that round and for the next {Barbarian level + CON modifier} rounds, the barbarian adds his Constitution modifier to to-hit rolls and his Constitution modifier and total Barbarian levels to any damage he deals. A barbarian who does not suffer a negative Intelligence modifier may voluntarily end his Rage at will. When a barbarian enters a Rage, he regains his Barbarian levels in Hit Points. At 5th level, the barbarian also adds half his Strength modifier (rounded down) to to-hit rolls and to damage rolls.