Tunneler. The worm can burrow through solid rock at half its Burrow Speed and leaves a 25-foot-diameter tunnel in its wake.

Internal Furnace. Fierce heat burns within the Burrower, making it immune to fire and cold damage. Any creature that starts its turn within 15 feet of the worm takes 1d10 fire damage and is set on fire.

Inexorable. Regardless of any status effects, this creature cannot be prevented from taking actions and its movement cannot be reduced below 10ft. Other elements of such status effects still take hold.

Bite. Melee Attack Roll: +14, reach 30 ft. Hit: 44 (10d6 + 9) Piercing damage. This attack targets all creatures in a 10ft radius sphere centred on a point within range. If the targets are Large or smaller creatures, they have the Grappled condition (escape DC 19), and the Restrained condition until the grapple ends. The worm chooses the point to centre this attack on using its action, however the attack is only resolved at initiative count 1.

Swallow. Strength Saving Throw: DC 19, one Large or smaller creature Grappled by the worm (it can have up to three creatures swallowed at a time). Failure: The target is swallowed by the worm, and the Grappled condition ends. A swallowed creature has the Blinded and Restrained conditions, has Total Cover against attacks and other effects outside the worm, and takes 17 (5d6) Fire damage at the start of each of the worm’s turns.

If the worm takes 20 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, each of which falls in a space within 20 feet of the worm and has the Prone condition. If the worm dies, any swallowed creature no longer has the Restrained condition and can escape from the corpse using 20 feet of movement, exiting Prone.

The Abyssal Burrower can take 3 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. It regains spent legendary actions at the start of its turn.

Baleful Gaze. The worm trains a number of its vile eyes on a creature within 60ft. That creature must make a DC 15 Wisdom saving throw or have its speed reduced to 0 until the end of its next turn. This legendary action cannot be repeated during the same round.
Stoke Furnace. Each creature within 30ft must make a DC 15 Constitution saving throw or take 1d10 fire damage and be set alight.
Seismic Thrash. Each creature within 30ft must make a DC 15 Strength saving throw or take 1d4 bludgeoning damage and be knocked prone