Multiattack. The serpent makes two attacks: one with its bite and one to constrict.

Bite. Melee Weapon Attack: +10 to hit, reach 10 ft., one target. Hit:22 (3d10 + 6) piercing damage.

Constrict. Melee Weapon Attack: +10 to hit, reach 5 ft., one Large or smaller creature. Hit:17 (2d10 + 6) bludgeoning damage plus 17 (2d10 + 6) slashing damage. The target is grappled (escape DC 16) if the serpent isn't already constricting a creature, and the target is restrained until this grapple ends.

Swallow. The serpent makes one bite attack against a Medium or smaller target it is grappling. If the attack hits, the target is also swallowed, and the grapple ends. While swallowed, the target is blinded and restrained, it has Cover against attacks and other effects outside the serpent, and it takes 21 (6d6) acid damage at the start of each of the serpent's turns. A serpent can have only one creature swallowed at a time.

If the serpent takes 30 damage or more on a single turn from the swallowed creature, the serpent must succeed on a DC 14 Constitution saving throw at the end of that turn or regurgitate the creature, which falls prone in a space within 10 feet of the serpent. If the serpent dies, a swallowed creature is no longer restrained by it and can escape from the corpse by using 15 feet of movement, exiting prone.