Source: Baldur's Gate - Descent into Avernus

Weapon (warhammer), legendary (requites attunement)

Matalotok, the Frost Father, is an ancient hammer fashioned by Thyrm, the god of frost giants. The favored weapon of the demon lord Kostchtchie, Matalotok is frigid to the touch and wreathed in mist.

You are immune to cold damage while holding Matalotok. Whenever it deals damage to a creature, the hammer radiates a burst of intense cold in a 30-foot-radius sphere. Each creature in that area takes 10 (3d6) cold damage.