Annelidast

Deep Earth Dweller. The annelidast doesn't require air.

Primordial Aura. A creature that starts its turn within 30 feet of the annelidast must succeed on a DC 17 Constitution saving throw or take 7 (2d6) poison damage. A creature that finishes a long rest within 1,000 feet of the annelidast must succeed on a DC 17 Constitution saving throw or develop tumors across its body and become poisoned until the tumors are removed by the greater restoration spell or similar magic.

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

Multiattack. The annelidast makes three Slam attacks.

Slam. Melee Weapon Attack: +12 to hit, reach 10 ft., one target. Hit: 21 (2d12 + 8) bludgeoning damage, and if the target is a creature, it must succeed on a DC 17 Strength saving throw or be pushed up to 15 feet away from the annelidast. A creature restrained by the annelidast automatically succeeds on this saving throw.

Crush (Recharge 5-6). The annelidast launches most of its massive bulk into the air and crashes down on the ground in a 20-foot line that is 10 feet wide. It then retracts itself into a space along that line and can occupy the space of one or more Large or smaller creatures. Each creature in the line must make a DC 17 Dexterity saving throw. On a failure, a creature takes 45 (7d12) bludgeoning damage, is knocked prone, and is restrained while it shares a space with the annelidast, as it is pinned beneath the annelidast's body. On a success, a creature takes half the damage, isn't knocked prone, and is pushed out of the annelidast's space into an unoccupied space of the creature's choice within 5 feet of the annelidast. A creature, including the restrained creature, can take its action to free the restrained creature by succeeding on a DC 17 Strength check. If the annelidast burrows along the surface of the earth at least 20 feet before using this action, creatures in the line have disadvantage on the saving throw.

Defensive Recoil. When the annelidast takes damage, it can make one Slam attack against a creature restrained by it.