Nightcrawler

Tunneler. The Nightcrawler can burrow through solid rock at half its Burrow Speed and leaves a 10-foot-diameter tunnel in its wake. Creatures in the path of its burrow must succeed on a DC 20 Dexterity saving throw or take 22 ((4d10) bludgeoning damage and be knocked prone. Non-undead creatures that start their turn in a tunnel take 10 ((3d6) necrotic damage from the corrupted shadow it leaves behind. A Nightcrawler's tunnels are considered difficult terrain. 

Unusual Nature. The Nightcrawler doesn’t require air, food, drink, or sleep.

Undead Fortitude. If damage reduces the Nightcrawler to 0 hit points, it must make a Constitution saving throw with a DC of 5 + the damage taken, unless the damage is radiant or from a critical hit. On a success, it drops to 1 hit point instead.

Sunlight Weakness. While in sunlight, the Nightcrawler has disadvantage on attack rolls, saving throws, and ability checks, and it cannot use its Spellcasting trait.

Magic Resistance. The Nightcrawler has advantage on saving throws against spells and other magical effects.

Aura of Darkness. Nonmagical light within 60 feet of the Nightcrawler is extinguished. Magical light of 3rd level or lower is dispelled, and higher-level light spells will require the spellcaster to succeed on a DC 18 Constitution saving throw to persist.

Life Eater. A creature dies if reduced to 0 hit points by the Nightcrawler and can’t be revived except by a wish spell.

Multiattack. The Nightcrawler makes two attacks, one with its Bite and one with its Tail Slam.

Bite. Melee Attack Roll: +14, reach 10 ft. Hit: 30 (4d10 + 9) Piercing damage plus 18 (4d8) Necrotic damage. If the target is a Large or smaller creature, it has the Grappled condition (escape DC 19), and it has the Restrained condition until the grapple ends.

Tail Slam. Melee Weapon Attack Roll: +15, reach 20 ft. Hit: 25 (3d12 + 9) Bludgeoning damage plus 13 (3d8) Necrotic damage.

Summon Undead (Recharge 5–6). Summons 1d8 shadows or 1d4 wraiths. They act immediately after the Nightcrawler and last 1 minute, until they or the Nightcrawler die, or until dismissed.

Swallow. Strength Saving Throw: DC 19, one Large or smaller creature Grappled by the Nightcrawler (it can have up to three creatures swallowed at a time). Failure: The target is swallowed by the worm, and the Grappled condition ends. A swallowed creature has the Blinded and Restrained conditions, has Total Cover against attacks and other effects outside the worm, and takes 35 ((10d6) necrotic damage at the start of each of the Nightcrawler's turns.

If the Nightwalker takes 50 damage or more on a single turn from a creature inside it, the worm must succeed on a DC 20 Constitution saving throw at the end of that turn or regurgitate all swallowed creatures, each of which falls in a space within 10 feet of the Nightwalker and has the Prone condition. If the Nightwalker dies, any swallowed creature no longer has the Restrained condition and can escape from the corpse using 30 feet of movement, exiting Prone.