Immutable Form. The golem is immune to any spell or effect that would alter its form.

Magic Resistance. The golem has advantage on saving throws against spells and other magical effects.

Magic Fists. The golem's slam attacks are magical.

Steam Powered. Whenever th golem gets takes fire damage, it gains that many temporary Hit Points. While it has those hipoints, it's speed is doubled, it can make a slam attack as a bonus action, and Unleash Fire has (recharge 1-6) 

Billowing Smoke. Whenever a creature starts its turn within 10ft. of the golem, they must make a DC 21 Constitution saving throw, on a failure they become poisoned. The creature can repeat the save on each of it's turns, ending the effect on a success, becoming immune to the smoke for 24 hours. 

Fiery Core. The golem sheds bright light for 30ft. and Dim light for another 50ft. 

 Multiattack: The golem makes 2 slam attacks

Slam. Melee Weapon Attack: +16 to hit, reach 10 ft., one target. Hit: 45 (8d8 + 9) Bludgeoning damage + 16 (3d10) fire damage

Unleash Fire (recharge 6). The golem opens its hatch to it's core and releases a massive eruption of flames. Each creature in a 60ft. Cone must make a DC 21 Dexterity Saving Throw, on a failure, the creature takes 66(12d10) fire damage, or half as much on a succesful save