A ring with a trigger that activates when the wearer makes a fist. When activated the ring becomes brass knuckles. 

When activated the wearer takes 1d4 damage and must make a WIS 15 save or enter a bloodthirsty rage for 1 minute where they attack the nearest thing then loses the rage but stay aggressive until unequipped

1d12 Thunder damage + dex modifier for melee or 15' cone 1d8+ dex thunder damage (dex save of 13) spell attack