Bridge Guardian

Fire Absorption: Whenever the guardian is subjected to fire damage, it takes no damage and instead regains a number of hit points equal to the fire damage dealt.

Regeneration: The guardian regains 15 hit points at the start of its turn due to the Fire Elemental within. 

Immutable Form: The guardian is immune to any spell or effect that would alter its form.

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

Magic Weapons: The guardian's weapon attacks are magical.

Superheated Body: Any physical contact with the guardian causes 5 (1d10) fire damage.

Water Susceptibility. The guardian takes 3 (1d6) Cold damage for every 5 feet the elemental moves in water or for every gallon of water splashed on it. It also becomes vulnerable to bludgeoning damage until it's next turn.

Superheated. If the guardian is immersed in lava, it's body softens and it becomes vulnerable to bludgeoning damage.

Multiattack: The golem makes three Slam attacks.

Slam: Melee Weapon Attack: +9 to hit, reach 10 ft., one target. Hit: 20 (3d8 + 7) bludgeoning damage.

Fire Breath (Recharge 5-6). The guardian exhales flame in a 40-foot cone. Each creature in that area must make a DC 18 Dexterity saving throw, taking 50 (10d8+5) fire damage on a failed save, or half as much damage on a successful one.

Legendary Action Uses: 3. Immediately after another creature’s turn, the guardian can expend a use to take one of the following actions. The guardian regains all expended uses at the start of each of its turns.

Slam (Costs 1 Action). Melee Weapon Attack: +9 to hit, reach 10 ft., one target. Hit: 20 (3d8 + 7) bludgeoning damage.

Disengage (Costs 1 Action). The Guardian takes the disengage action.

Fire Breath (Recharge 5-6, Costs 2 Actions). The guardian exhales flame in a 40-foot cone. Each creature in that area must make a DC 18 Dexterity saving throw, taking 50 (10d8+5) fire damage on a failed save, or half as much damage on a successful one.