Bone Golem

Berserk: Whenever the bone golem starts its turn with 30 HP or fewer, roll a d6. On a 6, the golem goes berserk. On each turn 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 smaller than itself. Once the golem goes berserk, it continues until it is destroyed or regains all of its HP. The golem's creator, if within 60 feet of the berserk golem, can calm it if the golem can hear its creator and the creator spends an action to make a DC 15 Charisma (Persuasion) check.

False Appearance: While the bone golem remains motionless, it cannot be identified as anything but a pile of meat and bones. A detect magic or identify spell will reveal that the pile contains transmutation magic but not the nature of the construct.

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

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

Magic Weapons: The bone golem's attacks are considered magical.

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

Construct Nature: The golem does not require air, food, drink, or sleep.

Multiattack: The bone golem makes two Bone Claw attacks.

Bone Claw: Melee Weapon Attack: +10 to hit, reach 5 ft., one target. Hit: 13 (1d12 + 6) slashing damage plus 7 (2d6) necrotic damage. 

Hideous Shriek (Recharge 5-6): Dozens of skulls cry out in a unified wail that weakens even the bravest foes. All creatures within 60 feet of the bone golem that can hear it must make a DC 15 Wisdom saving through or become frightened. Affected creatures can make a saving throw to end this condition at the end of their turn.