Tectonic Plate

Armour (breastplate, half plate, or full plate), uncommon
  Forged by fusing the heart of a fire elemental within the rocky plates of an earth elementals torso, the cracks of this black, basalt armour shimmer with barely-concealed heat. Remarkably warm and comfortable, this armour takes immediate retribution against any creature that would do its wearer harm.
  This armour is perpetually warm to the touch. You have advantage on any checks or saving throws made to resist the effects of a cold environment.
  Eruption. When you are hit by a melee attack, the armour sprays a spurt of magma at the attacker, dealing 1d4 fire damage to the attacker. After this effect has triggered, it can’t trigger again until the start of your next turn. In addition, roll a d6. On a 1, the armour goes dark and this effect can’t trigger again until the following dawn.
  Rare variant: Increase the fire damage to 1d8 and the discharge d6 to a d8.
  Very rare variant: Increase the fire damage to 1d12 and the discharge d6 to a d10.
  Legendary variant: Increase the fire damage to 2d8 and the discharge d6 to a d12.