Multiattack. One Bite attack and one Tail Fin attack.

Bite. Melee Weapon Attack: +10 to hit, 10 ft., one target, 19 (3d8+6) piercing damage. If target is a Large or smaller creature: swallowed by the worm (DC 16 Dex negates). A swallowed creature is blinded and restrained it has total cover vs. attacks and other effects outside the worm and it takes 14 (4d6) acid at the start of each of the worm's turns. If the worm takes 25+ damage on a single turn from a creature inside it worm must make DC 18 Con save at the end of that turn or regurgitate all swallowed creatures which fall prone in a space within 10 ft. of the worm. If worm dies a swallowed creature is no longer restrained by it and can escape the corpse by using 15 ft. of movement exiting prone.

Tail Fin. Melee Weapon Attack: +10 to hit, 10 ft., one target, 16 (3d6+6) slashing damage + 16 (3d6+6) bludgeoning damage.

Red Acid Spume (Recharge 5-6). Exhales crimson stomach acid in 30' cone if worm is underwater or 50' line × 5 ft. wide if above water. Each creature in area: 31 (9d6) acid (DC 16 Dex half).