Coral Golem

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.

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

Ecosystemic Body. The golem is made of a small ecosystem. When the golem drops to 0 hit points, unless it took acid, necrotic, or poison damage in the last minute, it slowly rebuilds over the course of 1d8 days.

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. 

Fish Swarm. Ranged weapon attack: +3 to hit, reach 15 ft., one target. Hit: 16 (2d10 + 5) piercing damage. If the target is a creature, it must succeed on a DC 15 Constitution saving throw or take an additional 4 (1d8) necrotic damage. You heal for twice the necrotic damage dealt this way.

Haste (Recharge 5–6). Until the end of its next turn, the golem magically gains a +2 bonus to its AC, has advantage on Dexterity saving throws, and can use its slam attack as a bonus action.