Adult Flintworm

Radiant Mirror. If the worm takes radiant damage, each creature within 10ft. of it takes that damage as well.

Thermal Exhaust. each creature within a 10ft. radius of the worm takes 13 (3d8) fire damage at the start of the worm's turn.

Illumination. The worm sheds dim light in a 15ft radius.

Tunneler. The worm can burrow through solid rock at half of its burrowing speed and leaves a 5-foot-diameter tunnel in its wake.

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

Barbed Tentacles. Melee Weapon Attack: +8 to hit, reach 10 ft., one target. Hit: 17 (3d6 + 7) piercing damage, and the target is grappled (escape DC 15). until this grapple ends, the target is restrained and takes 13 (3d8) fire damage at the start of each of the worm's turns. The tentacles can grapple only one creature at a time.   

Bite. Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 20 (3d8 + 7) piercing damage. If the target is a Small or smaller creature, it must succeed on a DC 15 Dexterity saving throw or be swallowed by the worm. A swallowed creature is blinded and restrained, has total cover against attacks and other effects outside the worm, and takes 13 (3d8) 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 of it, the worm must succeed a DC 21 Constitution saving throw at the end of that creature's turn or regurgitate all swallowed creatures, which fall prone in a space within 10 ft. of the worm. If the worm dies, a swallowed creature can escape from the corpse by using 20 ft. of movement, exiting prone.