Purple Worm

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

The purple worm makes two attacks: one with its bite and one with its stinger.

Melee Weapon Attack: +14 to hit, reach 10 ft.. Hit: 22 (3d8 + 9) piercing damage.
If the target is a Large or smaller creature, it must succeed on a DC 19 Dexterity saving throw or be swallowed by the purple worm. A swallowed creature is blinded and restrained, it has total cover against attacks and other effects outside the purple worm, and it takes 21 (6d6) acid damage at the start of each of the purple worm's turns.
If the purple worm takes 30 damage or more on a single turn from a creature inside it, the purple 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 purple worm. If the purple 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.

Melee Weapon Attack: +14 to hit, reach 10 ft.. Hit: 19 (3d6 + 9) piercing damage.
The target must make a DC 19 Constitution saving throw, taking 42 (12d6) poison damage on a failed save, or half as much damage on a successful one.