Source: Legends of Runeterra - Dark Tides of Bilgewater

Wondrous item, uncommon (requires attunement)

This metal helmet is set with crystals that change color when exposed to elemental energy. While wearing this helm, if you are the target of an attack which would deal acid, cold, fire, lightning, or thunder damage you can use a reaction to adapt to that damage type. While adapted, you have resistance to that damage type. The helm returns to its normal state after ten minutes.