Werefang

Wondrous item, very rare (requires attunement)

This pendant is created from the tooth or teeth of a were-creature. It will not function during the day, but at night you can call upon it to transform you into the same kind of werecreature to which the tooth belonged. This effect is otherwise treated as a shapechange spell, lasting 2 minutes. After the first minute, you must make a Charisma saving throw at the start of each round to avoid going berserk, using every action to attack the nearest creature.

This effect can be dispelled prematurely by magic or if you drop to 0 hit points, but will otherwise last for the full 2 minutes.