Copper Golem

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

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 slam attacks.

Slam. Melee Weapon Attack: +10 to hit, reach 5 ft., one target. Hit: 13 (2d8 + 4) bludgeoning damage.

Lightning breath (Recharge 6). The golem exhales lightning in a 15-foot cone. Each creature in that line must make a DC 15 Dexterity saving throw, taking 38 (7d10) lightning damage on a failed save, or half as much damage on a successful one.