Altar Flame Golem

If the golem takes cold damage or is doused with at least three gallons of water, it has disadvantage on attack rolls and ability checks until the end of its next turn.

When the altar flame golem is reduced to 0 hp, it explodes into shards of hot stone and fire. Each creature within 15 feet of it must make a DC 16 Dexterity saving throw, taking 28 (8d6) fire damage on a failed save, or half as much damage on a successful one. An altar flame golem is not immune to the fire damage of another altar flame golem's death burst and doesn't absorb it.

While the golem remains motionless, it is indistinguishable from an altar bearing an eternal flame.

Whenever the golem is subjected to fire damage, it takes no damage and instead regains a number of hp equal to the fire damage dealt.

The golem is immune to any spell or effect that would alter its form.

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

The golem's weapon attacks are magical.

The altar flame golem makes two slam attacks.

Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 13 (2d8 + 4) bludgeoning damage plus 11 (2d10) fire damage.

Flame Breath (Recharge 5?6). The golem breathes fire in a 30-foot cone. Each creature in that area must make a DC 16 Dexterity saving throw, taking 36 (8d8) fire damage on a failed save, or half as much damage on a successful one.