Chaos Golem

Chaos Absorption. Whenever the golem is subjected to damage, it takes no damage and instead generates a random spell effect.

Berserk. Whenever the golem starts its turn with 60 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.

Chaotic Magic reflection. When a spell or other magical effect hits the Chaos Golem the spell or effect is negated and a random spell is cast by the Chaos golem. Roll a d6. on a 1-3 the spell targets the original caster, on a 4 a party member of the caster, on a 5 a random object 5d6 times 10 feet away, on a 6 it targets the chaos golem. 

Magic Weapons. The golem’s attacks are magical.

Multiattack. The golem makes two slam attacks.

Slam. Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 16 (2d10 + 5) bludgeoning damage. If the target is a creature, it must succeed on a DC 19 Constitution saving throw or have its hit point maximum reduced by an amount equal to the damage taken. The target dies if this attack reduces its hit point maximum to 0. The reduction lasts until removed by the greater restoration spell or other magic. Damage may be replaced by a random spell effect. DM's choice.

Random spell effects Roll 1d568