Bronze sentinel

Molten core. 

Whenever the Sentinel takes 15 or more damage from a single attack, molten bronze sprays from a crack.

Each creature within 5 feet must succeed on a DC 14 Dexterity save or take 5 (1d10) fire damage.

Divine Guardian

The Sentinel has advantage on saving throws against spells.

Cracked Armor

When reduced below 55 HP, the bronze shell fractures.

While bloodied:

• Speed increases to 40 ft.
• It gains advantage on melee attack rolls.
• Molten Core triggers after only 10 damage from a single hit.

Multi attack. The bronze sentinel makes two bronze slam attacks.

Bronze slam. Melee Weapon Attack: +8 to hit, reach 4 ft., 1 target. Hit: 11 (2d6 + 5) [bludgeoning] damage. 

If the same creature is hit in the same turn by 2 bronze slams, it must succeed a dc 13 strength or constitution saving throw or be knocked prone.

Molten spear. Ranged Weapon Attack: dexterity saving throw, dc 14, range 30 ft., 12 (4d6) [fire] damage. On a fail, restrained until end of next turn.

Bronze stomp. (recharge 6) Every creature within 10 feet must make a DC 15 Dexterity saving throw.

Failure:

• 13 (3d8) bludgeoning damage
• Knocked prone.

Success:

• Half damage.

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The Sentinel has 2 Legendary Actions, choosing from the options below. Only one option may be used at a time, at the end of another creature's turn.

Move (Costs 1 Actions). Moves up to half its speed without provoking opportunity attacks.

Hammering Blow. (Costs 2 actions) Makes one Bronze Slam attack.

Molten Pulse (Costs 2 Actions). The bronze veins flare, every creature within 5 feet takes 4 (1d8) fire damage.