Brick Golem

Lightning  Absorption. Whenever the golem is subjected to lightening damage, it takes no damage and instead regains a number of hit points equal to the acid damage dealt.

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

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

Multiattack. The golem makes two fist attacks.

Fist. Melee Weapon Attack: +12 to hit, reach 5 ft., one target. Hit: 16 (2d10 + 5) bludgeoning damage. 

Tackle. Melee Weapon Attack: +12 to hit, reach 1 ft., one target: Hit: 15 (3D10+5). On a successful attack, the creature must make a DC17 Dexterity save or be knocked back 10" and prone for 1D43 rounds. A successful save will result in the creature being knocked back 10" but not prone. 

Regen (Recharge 5–6). Each turn, the golem  gains 2hp when the golem's hit points have been reduced to half or below.