Corrupted Avenger

Weapon (<type>), Unique, Requires attunement

This ancient forgotten <weapon type> was once a mighty Holy Avenger. Its lack of use has caused it to magically decay over time, losing its potency and reverting almost completely into an ordinary magic weapon.

While you are attuned to this weapon, when you accomplish acts of greatness or heroism in battle with it, the weapon gains some of its power back, permanently gaining a +1 to attack and damage rolls. When this bonus reaches +3, the item returns to its former glory, transforming into a Holy Avenger.