Heated Body. A creature that touches the worm or hits it with a melee attack while within 5 feet of it takes 14 (4d6) fire damage. At the start of each of the worm's turns, it deals 14 (4d6) fire damage to any creature that is grappling it and any object that is restraining it.

Lava Tunneler. The worm can burrow through lava, magma, and solid rock. Its tremorsense treats lava and magma as if it were solid stone.

Molten Tunnels. When the worm burrows, it leaves a 10-foot diameter tunnel in its wake. The sides of this tunnel remain burning hot for 10 minutes, after which the tunnel collapses. When a creature enters the tunnel for the first time on a turn or starts its turn inside the tunnel, it takes 11 (2d10) fire damage.

Siege Monster. The worm deals double damage to objects and structures, and it treats immunity to fire damage as resistance to fire damage when it hits objects or structures with its bite attack.

Multiattack. The worm makes two attacks.

Bite. Melee Weapon Attack: +15 to hit, reach 10 ft., one target. Hit: 28 (3d12 + 9) piercing damage plus 28 (8d6) fire damage. If the target is a creature or a flammable object, it ignites. Until a creature takes an action to douse the fire, the target takes 11 (2d10) fire damage at the start of each of its turns. In addition, if the target is a Large or smaller creature, it must succeed on a DC 20 Dexterity saving throw or be swallowed by the worm.

While swallowed, the creature is blinded and restrained, it has total cover against attacks and other effects outside the worm, and it takes 21 (6d6) acid damage plus 21 (6d6) fire damage at the start of each of the lava worm's turns.

If the worm takes 40 damage or more on a single turn from a creature inside it, the worm must succeed on a DC 21 Constitution saving throw at the end of that turn or regurgitate all swallowed creatures, which fall prone in a space within 10 feet of the worm. If the worm dies, a swallowed creature is no longer restrained by it and can escape from the corpse using 15 feet of movement, exiting prone.

Flame Spit. Ranged Spell Attack: +13 to hit, range 120 ft., one creature. Hit: 55 (8d10 + 7) fire damage. If the target is a creature or a flammable object, it ignites. Until a creature takes an action to douse the fire, the target takes 11 (2d10) fire damage at the start of each of its turns.

Breath Weapon (Recharge 5–6). The worm sprays molten rock in a 60-foot cone. Each creature in that area must make a DC 21 Dexterity saving throw, taking 55 (10d10) fire damage on a failed save, or half as much damage on a successful one. On a failed saving throw, a creature is also restrained by the hardening rock. A creature can make a DC 21 Strength (Athletics) check as an action, freeing itself or a creature within reach from the rock on a success. The rock has AC 17 and 60 hit points, and it is immune to fire, poison, and psychic damage, but it is vulnerable to any damage dealt by a lava worm's bite attack.