Brain Worm

Tunneler. The worm can burrow through solid rock at half its burrow speed and leaves a 10-foot-diameter tunnel in its wake.

Magic Resistance. The brain worm has advantage on saving throws against spells and other magical effects.

Sturdy. If the Brain Worm were to be pushed by an attack or spell, it can chose to make a Con saving throw (level of opponent +1), getting pushed no distance on a success and half on a fail

Innate Spellcasting (Psionics). The brain worm's innate spell casting ability is Intelligence (spell save 15). It can innately cast the following spells without components:

At will: Levitate, mind sliver, blade ward. 

Multiattack. The worm makes two attacks: one with its bite and one with its stinger or two with its tentacles.

Bite. Melee Weapon Attack: +15 to hit, reach 15 ft., one target. Hit: 23 (3d8 + 10); piercing damage and 6 (1d8+1) psychic damage. 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. A swallowed creature is blinded and restrained, it has total cover against attacks and other effects outside the worm, and it takes 32 (8d6); acid damage at the start of each of the worm's turns.

If the worm takes 45 damage or more on a single turn from a creature inside it, the worm must succeed on a DC 22 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 20 feet of movement, exiting prone.

Tail Stinger. Melee Weapon Attack: +15 to hit, reach 10 ft., one creature. Hit: 20 piercing damage, and the target must make a DC 20 Wisdom saving throw, taking 42 (12d6) psychic damage on a failed save, or half as much damage on a successful one.

Tentacles. Melee Weapon Attack: +15 to hit, reach 15 ft, one creature. Hit: 52 (8d10 + 4) psychic damage. If the target is large or smaller, it is grappled (escape DC 18) and must succeed on a DC 18 Intelligence saving throw or be stunned until this grapple ends. Until the grapple ends the worm does an extra 4d8 damage with its bite attack to the restrained creature.