Lava Form. A creature or object that touches the elemental or hits it with a melee attack while within 5 feet of it takes 10 (3d6) fire damage. A creature that ends its turn within 5 feet of the elemental takes 5 (1d10) fire damage. At the end of the elemental's turn, it ignites all flammable objects within 5 feet of it that aren’t being worn or carried.

Cooled Lava. If the elemental takes cold damage, it partially cools to stone; its speed is reduced by 20 feet until the end of its next turn, and its Lava Form trait doesn't function for that duration.

False Appearance. While the elemental remains motionless, it can choose to suppress its Lava Form trait, causing it to be indistinguishable from a normal (albeit huge) boulder.

Illumination. The elemental sheds bright light in a 30-foot radius and dim light for an additional 30 feet while its Lava Form trait is functioning.

Molten Tunnels. When it burrows, the elemental leaves a 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 elemental deals double damage to objects and structures.

Water Susceptibility. For every 5 feet the elemental moves in water, or for every gallon of water splashed on it, it takes 1 cold damage.

Dual Nature. The elemental counts as an elemental of both the earth and fire sub-types (primary earth).

Multiattack. The elemental makes two smash attacks.

Smash. Melee Weapon Attack: +8 to hit, reach 10ft., one creature. Hit: 16 (2d10 + 5) bludgeoning damage plus 21 (6d6) fire damage, and the target must succeed on a DC 16 Strength saving throw or be knocked prone. If the target is not flying, on a hit or a miss, the area of ground in a 15–foot diameter circle centered beneath the target becomes filled with lava. The area is difficult terrain for the next 10 minutes, and any creature who enters the area or ends its turn there takes 21 (6d6) fire damage.

Meteor Breath. The elemental spews tiny chunks of burning stone. Each creature in a 10-foot radius sphere centered on a point within 60 feet of the elemental must make a DC 16 Dexterity saving throw. On a failure, a target takes 10 (3d6) bludgeoning damage plus 10 (3d6) fire damage, or half as much damage on a success. Each object within the area that isn't being held or carried also takes full damage, and is ignited if it is flammable.