Ancient Purple Worm

Tunneler. The worm can burrow through solid rock at half its burrow speed and leaves a 20-foot-diameter tunnel in its wake.

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

Bite. Melee Weapon Attack: +12 to hit, reach 10 ft., one target. Hit: 22 (3d6 + 5) piercing damage. If the target is a Large or smaller creature, it must succeed on a DC 18 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 (4d6) acid damage at the start of each of the worm's turns.

If the worm takes 30 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 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: +12 to hit, reach 10 ft., one creature. Hit: 19 (2d6 + 5) piercing damage, and the target must make a DC 18 Constitution saving throw, taking (8d6) poison damage on a failed save, or half as much damage on a successful one.

Body Whip. Melee Weapon Attack: +12 to hit, reach 10 ft., one creature. In response to being attacked, the Worm can whip its body around to deal 19 (2d6+3) bludgeoning damage, and the target must make a DC 15 Strength saving throw or be moved 10 feet away from the Worm.

The worm has 4 action points each round.

Reposition (1 point). The worm uses its full Burrow speed to move underground.

Cracked Earth (3 points). The worm uses the strength of its tail to crumble the earth around it, creating difficult terrain within a 20 foot radius of the worm.

Digest (2 points). If the worm has a creature Swallowed, it can use this action to begin digesting the Swallowed creature. The Swallowed creature must make a DC 15 CON save, or take (2d6) acid and (2d6) bludgeoning damage.