Alchemical golem

Alchemical healing. When the golem takes acid damage from a spell it instead heals the amount of damage. 

Rupture. If a target deals at least 10 points of slashing damage or 10 points of piercing damage to the golem the direction they got hit from has a 5ft ft by 5 ft square of acid that lasts until the end of the golem's next turn. If a creature steps in the square then they take 6 (2d6) acid damage. 

Limited Magic Immunity. The golem can’t be affected or detected by spells of 6th level or lower unless it wishes to be. It has advantage on saving throws against all other spells and magical effects.

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.

Berserk. Whenever the golem starts its turn with 40 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 40 hit points or fewer, the golem might go berserk again.

Multi attack. The alchemical golem makes two slam attacks.

Slam. Melee Weapon Attack: +6 to hit, reach 10 ft., 1 target. Hit: 14 (2d8 + 6) bludgeoning damage plus 6 (2d6). 

Acid breath (recharges on 6). The golem exhales a 15 ft cone of acid and each creature within the radius must make a dc 16 Constitution save or take 15 (5d6) acid damage half as much on a successful save.