Blood Golem

Amorphous. The golem can move through a space as narrow as 1 inch wide without squeezing.

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.

Aversion of Cold. If the golem takes cold damage, it has disadvantage on attack rolls and ability checks until the end of its next turn.

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.

Amorphous Grapple. Whenever the golem uses an action to attempt to grapple a creature within 5ft of it, it can also attempt to grapple any other creature grapple-able by it within 5ft of itself or the first main targeted creature to be grappled.

Multiattack. The golem makes two siphoning slam attacks.

Siphoning Slam. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 13 (2d8 + 4) necrotic damage. The target's hit point maximum is reduced by an amount equal to the necrotic damage taken, and the golem regains hit points equal to that amount. The reduction lasts until the target finishes a long rest. The target dies if this effect reduces its hit point maximum to 0.