Coal Golem

Berserk. Whenever the golem starts its turn with 20 hit points or fewer, roll a d6. On a 6, the golem goes berserk. On each of its turns while berserk, the golem attacks the nearest creature it can see. If no creature is near enough to move to and attack, the golem attacks an object, with preference for an object smaller than itself. Once the golem goes berserk, it continues to do so until it is destroyed or regains all its hit points.

The golem's creator, if within 60 feet of the berserk golem, can try to calm it by speaking firmly and persuasively. The golem must be able to hear its creator, who must take an action to make a DC 15 Charisma (Persuasion) check. If the check succeeds, the golem ceases being berserk. If it takes damage while still at 20 hit points or fewer, the golem might go berserk again.

Fuel for the Fire. If the golem takes fire damage, attack rolls against it have advantage until the end of its next turn, but it gains a single charge of its fiery rebuke.

Douse the Fire. If the golem comes into contact with water, it takes damage. If the contact is from an attack, such as from a spell, it is an automatic critical hit. Otherwise, the golem takes 1d6 cold damage when the water touches it, and again at the end of every turn it is still in contact with it.

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

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

Multiattack. The golem makes two slam attacks.

Slam. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 5 (1d6 + 2) bludgeoning damage.

Fiery Rebuke. This Golem is highly flammable, and attacking it can be dangerous. When hit by a melee attack, the golem can use its reaction to expend 1 charge and cause its attacker to make a DC 12 dexterity saving throw. They take 7 (1d8 + 2) fire damage on a failed save or half as much on a successful one.

The golem has one innate charge, and gains one charge each time it takes fire damage, to a maximum of 3.