Ancient Snake

Tunneler. The snake can burrow through solid rock at half its burrow speed and leaves a 10-foot-diameter tunnel in its wake.

Multiattack. The snake makes two attacks: one with its tail and one with either its bite or constrict.

Bite. Melee Weapon Attack: +10 to hit, reach 10 ft., one target. Hit: 19 (3d8 + 6) piercing damage. If the target is a Large or smaller creature, it must succeed on a DC 17 Dexterity saving throw or be swallowed by the snake. A swallowed creature is blinded and restrained, it has total cover against attacks and other effects outside the snake, and it takes 14 (4d6) acid damage at the start of each of the snake's turns.

If the snake takes 30 damage or more on a single turn from a creature inside it, the snake 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 snake. If the snake dies, a swallowed creature is no longer restrained by it and can escape from the corpse by using 20 feet of movement, exiting prone.

Constrict. Melee Weapon Attack: +10 to hit, reach 10 ft., one creature. Hit: 19 (3d8 + 6) bludgeoning damage, and the target is grappled (escape DC 17). Until this grapple ends, the creature is restrained, and the snake can't constrict another target.

Tail. Melee Weapon Attack: +10 to hit, reach 10 ft., one creature. Hit: 16 (3d6 + 6) bludgeoning damage, and the target must make a DC 17 Strength saving throw or be knocked back and prone 15 feet.