Forgefire Warhammer

The head of this weapon is shaped like an anvil, and engravings of fire and arcane runes decorate it. You can use a bonus action to speak this magic weapon's command word, causing its head to glow red-hot. While red-hot, the weapon deals an extra 1d6 fire damage to any target it hits. The weapon's anvil-like head remains red-hot until you use a bonus action to speak the command word again or until you drop or sheathe the weapon. When you roll a 20 on an attack roll made with this weapon against a target holding or wearing a metal object, such as a metal weapon or metal armor, the target takes an extra 1d4 fire damage and the metal object becomes red-hot for 1 minute. While the object is red-hot and the creature still wears or carries it, the creature takes 1d4 fire damage at the start of each of its turns.