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

Legendary Resistance (3/Day). If Yolmar Ancient Purple Worm fails a saving throw, he can choose to succeed instead.

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

Bite. Melee Weapon Attack: +15 to hit, reach 20 ft., one target. Hit: 23 (3d8 + 10) 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 worm. A swallowed creature is blinded and restrained, it has total cover against attacks and other effects outside the worm, and it takes 25 (7d6) 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 30 ft., one creature. Hit: 20 (3d6 + 10) piercing damage, and 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.

Yolmar can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature’s turn. Yolmar regains spent legendary actions at the start of his turn.

Tail (Cost 2 Actions). Melee Weapon Attack: +15 to hit, reach 30 ft., one creature. Hit: 20 (3d6 + 10) piercing damage, and 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.

Tunnel. Yolmar burrows underground  causing each creature within 10 feet of Yolmar to succeed a DC 16 dexterity saving throw of take 21(2d10 + 10) bludgeoning damage and be knocked prone. Yolmar can then move up to its burrow speed.