A set of steel-blue bracers with streaks of lightning gold embedded throughout. 

The bracers grant resistance to lightning, but as a bonus action, you can slam your fists together to unclasp the "knucks" and change them into gaunlets that grant an extra 1d6 thunder damage on hit with unarmed strikes.

Only one effect can be active at a time. You can have resistance to lightning OR the extra 1d6 thunder damage. Never both.

Notes: Resistance: Lightning, Damage: Thunder, Barbarian, Monk, or Fighter