Bone golem

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

Berserk. Whenever the golem starts its turn with 75 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 60 hit points or fewer, the golem might go berserk again.

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

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. It can use its Soul Siphon in place of one Slam attack.

Slam. Melee Weapon Attack: +9 to hit, reach 5 ft., one target. Hit: 16 (2d10+5) piercing damage.

Soul Siphon. Melee Weapon Attack: +9 to hit, reach 5 ft., One creature. Hit: 16 (2d10+5) piercing damage plus 10 (3d6) necrotic damage. The target's hit point maximum is reduced by an amount equal to the necrotic damage taken, and the Bone Golem regains hit points equal to that amount. The reduction lasts until the target finishes a long rest. The target dies if this effect reduced its hit point maximum to 0.