Burrower. The worm can burrow through solid rock and leaves a tunnel of 8 feet wide in its wake. When it exits, each character in a 5ft distance must make a DC 15 Dexterity saving throw falling prone and taking 8 (2d6) bludgeoning damage on a fail, or just half as much damage on a save.

Deep Rock Appetite. If the worm borrows it eats rocks from the Underdark and regains 1d8 health. However, if it decides to forgo part of its Multiattack it can eat rocks around it for 2d8 health.

Split In Two. The worm can willingly split itself in half keeping itself intertwined with itself for the purposes of counting it as one enemy. Doing so it loses 2d10 health in the process but for the next 4 turns heals 10 (2d8) for each split, each turn.

Sulfuric Vulnerability. If struck with a sulfuric liquid/acid the worm must make a DC 20 Consitution save or regurgitate all swallowed creatures, which fall prone in a space within 15 feet of the worm. 

Multiattack. The worm makes three attacks: one with its bite and two with its acidic shot. It can forgo either the bite attack or two acidic shots to instead use the Deep Rock Appetite feature or the Split In Two feature.

Bite. Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 12 (2d8 + 2) piercing damage. If the target is a Medium or smaller creature, it must succeed on a DC 16 Dexterity saving throw or be swallowed by the worm. A swallowed creature is blinded and restrained, it has total cover against attacks and other effects outside the worm, and it takes 21 (6d6) acid damage at the start of each of the worm's turns.

If the worm takes 25 damage or more on a single turn from a creature inside it, the worm must succeed on a DC 20 Constitution saving throw at the end of that turn or regurgitate all swallowed creatures, which fall prone in a space within 15 feet of the worm. If the worm dies, a swallowed creature is no longer restrained by it and can escape from the corpse by using 25 feet of movement, exiting prone.

Acidic Shot. Ranged Weapon Attack: +10 to hit, reach 20 ft., one creature. Hit: 16 (2d6 + 9) acid damage.

Rock Cough. After eating a rock, the worm can cough it up throwing an array of rock chunks to fall upon a 5-foot radius centered on a point with a 20-foot range. Any creature in the area must make a DC 15 Dexterity saving throw taking 2d8 bludgeoning damage and falling prone on a failed save, or take half as much damage on success without falling prone.

Extreme Measures. If - at the end of a player's turn - the worm reaches less than 50 hit points, it can split into two completely separate Underdark Worms with each receiving 2/3rds of the hit points it originally had and regaining 2d6 health for each of the next 2 turns.