Bone Sandworm

Tunneler. The worm can burrow through sand and leaves a 10-foot-diameter tunnel of quicksand in its wake that lasts until the beginning of it's next turn. Creatures in it's wake must make a DC 13 Dexterity saving throw or be submerged in quicksand, which acts like difficult terrain. A creature who does not use their movement to get out of the quicksand on their turn is fully submerged in the sand at the end of their turn and considered blinded and restrained and begins to suffocate.

Ambusher. If the worm begins its turn submerged in sand and did not move in the previous round, it gets advantage on its first target and deals an extra (2d6) piercing damage.

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

Bite. Melee Weapon Attack: +12 to hit, reach 10 ft., one target. Hit: 20 (3d8 + 7) 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 worm. A swallowed creature has total cover against attacks and other effects outside the worm, but it takes 11 (3d6) piercing damage at the start of each of the worm's turns or whenever it uses movement while inside the worm.

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 can escape from the corpse by using 20 feet of movement, exiting prone.

Tail. Melee Weapon Attack: +12 to hit, reach 10 ft., one creature. Hit: 17 (3d6 + 7) piercing damage.