Tunneler. The worm can burrow through solid rock at half its burrow speed and leaves a 2.5-foot-diameter tunnel in its wake.

Multiattack. The worm makes two attacks: one with its Tail Stinger and one with its Bite.

Bite. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 7 (1d8 + 3) piercing damage, and if the target is a Small or smaller creature, it must succeed on a DC 13 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 3 (1d6) acid damage at the start of each of the worm’s turns.
If the worm takes 6 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 by using 5 feet of movement, exiting prone.

Tail Stinger. Melee Weapon Attack: Hit: +4 to hit, reach 10ft., one creature. Hit: 5 (1d4 + 3)piercing damage, and the target must make a DC 13 Constitution saving throw, taking 6 (2d6) poison damage on a failed save, or half as much damage on a successful one.

Tail Whip. When received a melee attack, the worm lashes his tail out to the attacker, reach 10ft. The target must must make an athletics check DC13, or be pushed away for 10ft and be stunned until the start of it's next turn

The worm has 3 legendary actions points

Perception check

Movement (1 point). The worm can slither away without provoking opportunity attacks.

Acid Spitter (2 points). The worm hurls out a bubble of acid in a 15-foot cone. Each creature in that area must make a DC 12 Dexterity saving throw, taking 3(1d6) acid damage on a failed save, or half as much damage on a successful one. If failed, the target starting his turn and covered in acid suffers 1 point of damage, it can use it's action to remove the acid. The worm will not use this if there are larvae in the target area.

Pool of Larvae. (3 points) The worm releases 1d4 larvae on the ground that attack immediately. They share initiative with the worm and will act before it in the next turns.

A larvae has 1 hitpoint, +0 to hit, 1 piercing damage + 1 poison damage, 10ft movement.
STR(+0) DEX(-1) CON(+2) INT(-4) WIS (-1) CHA(-2)