I'd suggest treating it as a curse, with the DM determining the means by which it can be lifted (personal sacrifice, strength of will, etc). For game mechanics, why not just use the Rage ability of the Badger in the MM as a base?
My suggestion:
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Hyrui Spirit: The character is posessed with a spirit of rage. When placed in a situation that threatens his life or the lives of loved ones, the character enters a berserk rage the following round, gaining +4 to Strength and Constitution, and -2 to AC. When enraged, the berserker may continue to fight until reaching -10 Hit Points, at which point he will drop dead.
The berserker will attack the source of the threat with reckless abandon, attempting to close with the target and engage in hand to hand or melee combat. The character cannot cast spells or make use of any non-continually functioning magic items (he would gain the benefits of a +2 sword, Boots of Leaping, or Bracers of Dexterity, but could not activate the Haste effect from a pair of Boots of Speed). He may not use any WIS, CHA, or INT-based skills, and may only use STR, DEX, or CON skills related to engaging the enemy (he could, for example, make a jump check to cross a gorge to reach his foe, or a climb check to scale a wall).
If the source of the threat is incapacitated (reduced to 0 HP or less), the berserker may make a Will save (DC 20). The berserker receives a +1 bonus to this saving throw for each round that has passed since the rage began. If he fails the roll, he seeks out the next-nearest PC or NPC as a target, and the rage continues (the bonus to the saving throw returns to +0 and must accumulate anew).
If a berserker is confronted by a loved one or ally, that character may make a Diplomacy skill check at DC 15. Success on this roll grants the berserker an opportunity to make a new saving throw, with an additional bonus of +1 for each 3 points by which the Diplomacy check was passed (minimum bonus of +1).
After leaving the berserk state, the character is considered to be Exhausted. His Strength and Constitution return to normal, and any additional hit points gained by the increase in CON are lost. If this results in the character's hit points being reduced below 0, he is considered to be stabilized. If his hit points are reduced to -10 or below, he dies as he leaves the rage.
Berserkers have a difficult time understanding emotions, and maintain a cold, emotionless presence. This gives them a -4 penalty to all CHA and CHA-based skill checks.
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It's a bit long, and the wording can be cleaned up (needs to have a note about how DMs can introduce the curse, and how PCs can work to have it temoved), but I think it's a fairly accurate conversion.