Adamantine. One of the strongest substances in existence, any critical hit against the golem becomes a normal hit. 

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 Weapons. The golem's weapon attacks are magical.

Mithril Weaponry. Attached to the golem's right arm is a thick band of solid mithril, the light weight of the metal and the enchantment upon it allows it to warp into different forms. As a bonus action the golem can change the mithril into any of the following.

Shield: Granting a +2 to the golem's AC

Sword: Granting the golem a new attack option

Arcane Ballista: Granting the golem a new attack option. Creates its own magical ammunition.

Gauntlet: Giving the golem advantage on its Slam attacks and any Strength checks it makes.

Legendary Saves (2/Day). If the golem fails a saving throw it may spend a legendary save save to pass the save instead.

Multiattack. The golem makes two Slam attacks or if its Mithril weaponry is a sword, one Slam and one Sword attack. If the Mithril Weaponry is a arcane ballista it may make one arcane ballista attack.

Slam. Melee Weapon Attack: +15 to hit, reach 5 ft., one target. Hit: 30 (5d8 + 8) bludgeoning damage.

Sword. Melee Weapon Attack: +15 to hit, reach 10 ft., one target. Hit: 34 (4d12 + 8) slashing damage.

Arcane Ballista. Ranged Weapon Attack: +10 to hit, range 120/480 ft., one target. Hit: 36 (6d10+3) force damage. The target must succeed a DC 25 Constitution saving throw or be stunned until the end of its next turn.

Chromatic Breath (Recharge 6). The golem exhales poisonous gas in a 20-foot cone. Each creature in that area must make a DC 25 Dexterity saving throw, taking 54 (12d8) choose acid, cold, fire, lightning, poison, or thunder damage (of the golem's choice) on a failed save, or half as much damage on a successful one.

Antimagic Cone (Recharge 6). The golem's eyes creates an area of antimagic, as in the antimagic field spell, in a 150-foot-cone. The cone lasts until the end of the golem's next turn and points in what ever direction the golem faces.

The golem has 2 legendary actions, selecting from the options below. It can take only one legendary action at a time and only at the end of another creature’s turn. The golem regains spent legendary actions at the start of its turn.

Attack (Costs 1 Action). The golem makes one Slam attack or it can make one Sword or Arcane Ballista attack if its Mithril Weaponry is in that form.

Charge (Costs 1 Action). The golem moves up to half its movement speed in a straight line. Any creatures in this line must make a DC 25 Strength save or take 17 (5d6) bludgeoning damage and be knocked prone. Taking half damage and not being knocked prone on a successful save.