Iron Golem

Iron Golem CR: 16

Large construct, neutral

Armor Class: 20 (natural armour)

Hit Points: 210 20d10+20

Speed: 30 ft

STR

24 +7

DEX

9 -1

CON

20 +5

INT

5 -3

WIS

11 +0

CHA

1 -5

Damage Immunities: Fire, Poison, Psychic, Bludgeoning, Piercing, and Slashing from nonmagical attacks not made by adamantine weapons

Condition Immunities: Charmed, Exhaustion, Frightened, Paralyzed, Petrified, Poisoned

Senses: Darkvision 120 ft, passive Perception 10

Languages: Understands the languages of it's creator but can't speak

Challenge Rating: 16

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.
Immutable Form. The golem is immune to any spell or effect that would alter it's 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.

Actions

Multiattack. The golem makes two melee attacks.
Slam. Melee Weapon Attack +13 to hit, reach 5 ft, one target. Hit 20 3d8+7 bludgeoning attack.
Sword. Melee Weapon Attack +13 to hit, reach 10 ft, one target. Hit 23 3d10+7 slashing damage.
Poison Breath (Recharge 6). The golem exhales poison gas in a 15 foot cone. Each creature in that are must make a DC 19 Constitution saving throw, taking 45 10d6 poison damage on a failed save, or half as much damage on a successful one.

Iron golems are golems created by many creatures who regularly work with metal. This makes them one of the hardier types of golem.