Elemental Heartstone

This smooth, pulsating stone seems to contain a swirling mass of elemental energy. While attuned to the Elemental Heartstone, you gain resistance to one type of elemental damage (fire, cold, lightning, or acid). Additionally, once per long rest, you can unleash a wave of elemental energy as an action, dealing 3d8 damage of the chosen elemental type in a 15-foot cone. Creatures in that area must make a Dexterity saving throw (DC 14) to take half damage. On a successful save, they take no damage. Additionally, when you deal the elemental damage with an attack or spell, you can add 1d4 damage of that type to the attack.