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

Multiattack. The golem makes two melee attacks.

Slam. Melee Weapon Attack: +13 to hit, reach 15 ft., one target. Hit: (3d12 + 10) bludgeoning damage.

Xulo can take 1 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature's turn. The aboleth regains spent legendary actions at the start of its turn.

Bonemelt Sprayer. The hellfire engine spews acidic flame in a 60-foot cone. Each creature in the cone must make a DC 18 Dexterity saving throw, taking (2d10) fire damage plus (4d8) acid damage on a failed save, or half as much damage on a successful one. Creatures that fail the saving throw are drenched in burning acid and take (1d10) fire damage plus (2d8) acid damage at the end of their turns. An affected creature or another creature within 5 feet of it can take an action to scrape off the burning fuel.

Lightning Flail. Melee Weapon Attack: +11 to hit, reach 15 ft., one creature. Hit:(3d8 + 5) bludgeoning damage plus (5d8) lightning damage. Up to three other creatures of the hellfire engine’s choice that it can see within 30 feet of the target must each make a DC 18 Dexterity saving throw, taking (5d8) lightning damage on a failed save, or half as much damage on a successful one.

Thunder Cannon. The hellfire engine targets a point within 120 feet of it that it can see. Each creature within 30 feet of that point must make a DC 18 Dexterity saving throw, taking (5d10) bludgeoning damage plus (2d12) thunder damage on a failed save, or half as much damage on a successful one.