Ancient Sandworm

Legendary Resistance (1/Day). If the worm fails a saving throw, it can choose to succeed instead.

Tunneler. The worm can burrow through solid rock at half its burrow speed and leaves a 25-foot-diameter tunnel in its wake.

Static Wake. While burrowing, the worms creates static discharge through the sand. Any creature that starts its turn within 15ft of the worm when it emerges from burrowing must make a DC 20 dexterity saving throw, taking 14 (4d6) lightning damage on a failed save, or half as much damage on a successful one. 

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

Bite. Melee Weapon Attack: +15 to hit, reach 15 ft., one target. Hit: 27 (3d10 + 10); piercing 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  18(5d6) 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 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 20 feet of movement, exiting prone.

Tail Stinger. Melee Weapon Attack: +15 to hit, reach 15 ft., one creature. Hit: 20 (3d6 + 10) piercing damage, and the target must make a DC 20 Constitution saving throw, taking 42 (12d6) poison damage on a failed save, or half as much damage on a successful one.