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.

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

Necrotic Absorption. Whenever the golem is subjected to necrotic damage, it takes no damage and instead regains a number of hit points equal to the necrotic damage dealt.

Magic Resistance. The golem has advantage on saving throws against spells and other magical effects.

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

Multiattack. The golem makes two slam attacks.

Slam. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 13 (2d10 + 7) bludgeoning damage.

Ribcage Prison: If both attacks hit the same creature, the creature is grappled (escape DC 18). The golem will then pull the creature into it’s ribcage, engulfing it.

While a creature is so engulfed, on it's action it will take damage: 17 (4d6 + 7) necrotic damage and the golem will heal that amount. 

The creature may escape by dealing 50 points of damage from inside the golem or with Str save DC 20