The "Dragonlord" novels (by Joanne Bertin) have some interesting items that could be adapted.

In the third book there's a lyre made from the wood of a tree planted over the grave of a cannibalistic bard. The wood picked up his spiritual essence.

You could do something similar as the back story - a weapon used in the tomb of a fallen Barbarian (or other) has absorbed his essence and makes the wielder Hulk-out.

If you want to make it more in depth, the dead guy had mixed levels of Warlock and Barbarian. As the trapped essence takes more life, it becomes more powerful (doing more damage and healing the user more [physically]) but it drives the wielder to greater acts of violence and depravity.