Heart of the Forge. Fire damage heals the golem for half of the damage it would have taken.

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

Repair Mode. When the Adamantine Golem is reduced to 0 hit points, it is not destroyed. Instead it enters a self repair mode and can't move or react. Every turn while it is repairing it regains 30 hit points. After it gains 30 or more hit points then it can move and react. If it is reduced to 0 hit points by an adamantine weapon then it is destroyed. 

Vulnerable to Dispelling. If dispel magic is successfully cast on the golem (DC 25), then the golem loses its resistance to nonadamantine weapons for 1d4 turns and can be destroyed if reduced to 0 hit points.

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

Magic Weapons. The golem's weapon attacks are magical.

Siege Monster. The Adamantine Golem deals double damage to objects and structures.

Adamantine Body. Critical hits count as normal hits.

Self Repair. The golem repairs itself, regaining 30 hit points.

Fist. Melee Weapon Attack: +15 to hit, reach 10 ft., 1 target. Hit: 25 (3d8 + 9) bludgeoning damage. 

The golem has 4 legendary actions and regains them at dawn.

Vent Release (Costs 2 Actions). The golem releases the heat from its vents in a 15 ft cone of hot air. Each creature in this zone must make a Dexterity saving throw, taking 17 (3d8) fire damage on a failed save and half as much on a successful save.

Golem Smash (Costs 1 Action). The golem slams both of its hands on the ground. Each creature withing 20 ft. of the golem must make a Dexterity saving throw. On a failed save the creature takes 17 (3d8) bludgeoning damage. The 20 ft. surrounding the golem becomes difficult terrain.