Crater Wurm

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

Breach. The crater wurm utilises its full movement. While doing so, it can enter large or smaller creatures’ spaces. Whenever the wurm enters a creature’s space, the creature must succeed on a DC 19 Strength saving throw. On a successful save, the creature can choose to be pushed to the side of the wurm’s space. A creature that chooses not to be pushed suffers the consequences of a failed saving throw. On a failed save, the creature takes 35 (10d6) bludgeoning damage, and is pushed 10 feet and knocked prone. After finishing its move, the wurm can make one bite attack against a prone target as a bonus action.

Bite. Melee Weapon Attack: +14 to hit, reach 10 ft., one target. Hit: 22 (3d8 + 9) 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 wurm. A swallowed creature is blinded and restrained, it has total cover against attacks and other effects outside the wurm, and it takes 21 (6d6) acid damage at the start of each of the wurm's turns.

Contaminated Gullet. A creature swallowed by a crater wurm gains one level of contamination each time it takes acid damage while inside the wurm.