Crab Golem

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.

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.

Poison Absorption. Whenever the golem is subjected to poison damage, it takes no damage and instead regains a number of hit points equal to the lightning damage dealt.

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

Multiattack. The golem makes two slam attacks.

Slam. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 13 (2d8 + 4) bludgeoning damage.

Arm Fling. Ranged weapon attack: -1 to hit, reach 30 ft., one target. Hit: 13 (3d8-1) bludgeoning damage. Upon hitting its target the arm explodes into a swarm of crabs that will attack for 2 rounds and then return to the main body, during these 2 rounds the golem will not be able to preform a multiattack.