Collasatron

Fire Absorption. Whenever the golem is subjected to fire damage, it takes no damage and instead regains a number of hit points equal to the fire damage dealt.

Burning Armor. Due to the fire giant soul stuck inside the heart of the golem the outer metallic layer of armor is constantly heated. Any melee attack against the golem deals 2d4 fire damage back to the attacker.

Clockwork Deconstruction. As the golem takes damage its body parts break off. Each 50 hp that is lost the golem will lose an arm, leg, or its head. Roll a d6 to decide which piece breaks off. If an arm is lost the golem cannot use its multi-attack. If its leg is lost is cannot use the stomp action, and if the head is lost it is considered blinded until it regains its head.

Clockwork Reconstruction. Due to the clockwork nature of the golem it can rebuild damaged parts of its body. At the beginning of its turn if the golem is missing a body part roll a d4 on an even number the body part is rebuilt and the golem regains 25 hp. Any lost abilities are returned to the golem. The DM decides what body part is rebuilt.

Multi-attack. The golem makes a greatsword attack and a punch attack.

Greatsword. Melee Weapon Attack: +13 to hit, reach 10 ft., 1 target. Hit: 15 (2d10 + 10) [slashing] damage. 

Punch. Melee Weapon Attack: +13 to hit, reach 5 ft., 1 target. Hit: 8 (2d6 + 10) [bludgeoning] damage. 

Stomp. The golem stomps the ground causing a shock wave all creatures within 30 ft roll a DC 16 Dexterity saving throw. On a failure a creature takes 4d8 bludgeoning damage and is knocked prone. On a success the creature takes half damage.