Limited Immutable Form. The golem is immune to any spell or effect that would alter its form unless it would align with its Changing Form reaction. Additionally, regarding his available Changing Form reaction, the following additional rules apply: being petrified or paralyzed turns it into its Rock form, being poisoned or dealt Necrotic damage turns it into its Poison form, being dealt Radiant damage turns it into its Lightning form, and being turned invisible or dealt Thunder damage turns it into its Air form.  

Elemental Absorption (Acid, Cold, Fire, Lava, Lightning, Poison, or Water Form). Whenever the golem is subjected to damage of its current form's damage type, it takes no damage and instead regains a number of hit points equal to the damage dealt. In Water form, it regenerates 1 hit point for every 5 ft. of movement through water or for every gallon of water splashed on it. 

Cold Aura (Cold Form Only). At the start of each of the golem’s turns, each creature within 5 feet of it takes 4 (1d8) cold damage, and liquids in the aura that aren't being worn/carried are frozen. A creature that touches the golem or hits it with a melee attack takes 9 (2d8) cold damage.

Fire Aura (Fire or Lava Form Only). At the start of each of the golem’s turns, each creature within 5 ft. of it takes 4 (1d8) fire damage, and flammable objects in the aura that aren’t being worn/carried ignite. A creature that touches the golem or hits it with a melee attack takes 9 (2d8) fire damage and will continue to take 3 (1d6) fire damage at the start of its turns until an action is taken to extinguish the fire.

If lava form, the range is 15 ft. for the Aura range and fire damage is doubled.

Elemental Resistance (Acid, Air, Fire, Lava, Lightning, Poison, or Water Form Only). While in Acid/Air/Fire/Lava/Lightning/Poison/Water Form, the golem has:

• resistance to BPS due to fire's natural movement or Acid/Air/Lava/Poison/Water form being like living liquid.
• can enter a hostile creature's space & stop there as well as move through a space as narrow as 1 inch wide without squeezing.
• immunity to being restrained, grappled, or knocked prone.
• Acid Form. Getting soaked or submerged in water neutralizes the corrosive aura trait until the start of the Golem’s next turn and glass is immune to him.
• Air Form. Dexterity becomes 19 (+4) and it's always invisible.
• Fire Form. Dexterity becomes 19 (+4) and takes 2 (1d4) cold damage for every 5 feet of movement through water or for every gallon of water splashed on it. 
• Lightning Form. Dexterity becomes 26 (+8), creatures attack it w/disadvantage due to it being too fast to hit, can't have its speed reduced, and has the Evasion feature (half damage on Dex Save failure, no damage on success). It's also immune to being knocked prone.
• Poison Form. Is poison gas with inner corrosive form, can choose Acid or Poison type for attacks. It's also flammable, so it can use the fire type option for the breath attack if external spark is available; also takes an additional 7 (2d6) fire damage from explosions caused by each instance of taking fire damage. 
• Water Form. Acid resistance and cold damage reduces its speed by 20 ft. until the end of its next turn due to freezing liquid.

Cold/Earth/Metal/Rock/Wood Form. While in this form, its Strength becomes a 22 (+6) and it has resistance to BPS. It also has the additional properties:

• Cold Form. Has resistance to fire and vulnerability to bludgeoning. It also has climbing speed due to the ability that ice has to stick to objects, particularly metal; due to this, it breaks pieces off of itself on things when it walks or climbs but immediately regenerates lost parts. The pieces left behind exert the golem's Cold Aura until destroyed (1 hp) or they melt when the golem uses its Changing Form reaction.
• Metal Form. Immune to any attacks using Metal & vulnerability to Acid.
• Wood Form. Immune to any attacks using Wood, has vulnerability to Fire, no resistance to Slashing, no immunity to Poison, and can take any properties of absorbed wood including resistance or immunity to Poison.

Putrid Aura (Poison Form Only). A creature that starts its turn within 10 ft. of the golem must succeed on a DC 15 Constitution saving throw or be poisoned until the end of its next turn.

Corrosive Aura (Acid Form Only). A creature that touches the golem or hits it with a melee attack takes 13 (2d12) acid damage. Additionally, nonmagical weapons made of metal that hits the Acid form corrodes, taking a permanent and cumulative −1 penalty to damage rolls after dealing damage; the weapon is destroyed once the penalty reaches -5. Nonmagical metal ammunition is destroyed after dealing damage.

Multiattack. The golem makes two Slam attacks or two Elemental Touch attacks (three if in Lightning Form).

Slam. Melee Weapon Attack: +8 to hit (+10 for Earth/Metal/Rock/Wood form), reach 5 ft., one target. Hit: 13 (2d8 + 4) bludgeoning damage plus 7 (2d6) damage of a type determined by the golem’s form (excluding base form): acid (Acid or Poison form), cold, poison, slashing (Metal form), fire (Lava form or Poison form if provided an external spark source), or bludgeoning (Air/Earth/Rock/Water/Wood form). Add an additional 2 bludgeoning damage if in Earth/Metal/Rock/Wood form and add an additional 7 (2d6) fire damage for Lava form.

Elemental Touch (Fire or Lightning form only). Melee Weapon Attack: +8 to hit (+12 for Lightning form), reach 10 ft., one target. Hit: 15 (2d10+4) fire or lightning damage plus 4 lightning damage for Lighting form.

Breath Attack (Recharge 5-6, Acid/Cold/Fire/Poison form only). The golem exhales corrosive acid, freezing ice, a torrent of fire, or poisonous gas (respective to its current form) in a 30-foot cone. Each creature in that area must make a DC15 saving throw (Dexterity for Acid or Fire, Constitution for Cold or Poison), taking 21 (6d6) acid/cold/fire/poison damage on a failed save, or half as much damage on a successful one. Cold Form: Additionally, a failed save results in being hindered by ice formations for 1 minute, or until it or another creature within reach of it uses an action to break away the ice. A creature hindered by ice has its speed reduced to 0.

Sword Burst (Recharge 5-6, Metal form only). The golem shoots sword shrapnel off its body in a 30-foot radius. Each creature in that area must make a DC15 Dexterity saving throw, taking 21 (6d6) slashing damage on a failed save, or half as much damage on a successful one.

Lava Throw (Lava form only). Ranged Attack: +8 to hit, range 30 ft., one target. Hit: 31 (6d8+4) fire damage, and the target must succeed on a DC 15 Constitution saving throw or have one nonmagical item of the golem’s choice that the target is carrying instantly melt or burn to cinders.

Lightning Burst (Lightning Form Only). The golem teleports by dissipating to an unoccupied space it can see within 30 feet of it. Each creature within 10 feet of the golem after it teleports this way takes 6 (1d12) lightning damage.

Shift (Earth or Water Form Only). The golem turns to shifting earth/water or reforms to Earth form/Water form. While it is in shifting earth/water form, the golem:

• cannot attack,
• has immunity to BPS due to shifting materials,
• provokes no opportunity attacks while using its movement.

Root/Uproot (Wood Form Only). The golem can root itself to the ground or uproot itself. While rooted, the golem can regain 1d10 hit points if rooted in soil and cannot move or be moved.

Changing Form. The golem immediately takes on one of the following forms upon damage/contact with the corresponding type: Acid, Air, Cold, Earth, Fire, Lava, Lightning, Metal, Poison, Rock, Water, or Wood, with Thunder damage/contact corresponding to its Air form, Necrotic damage/contact corresponding to its Poison form, and Radiant damage/contact corresponding to its Lightning form. This form determines the creature’s AC, damage resistances, damage immunities, condition immunities, traits, and attacks. If it doesn't take damage or come into contact from any of those damage types, it remains in base form.