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

Legendary Resistance (3/Day). If the Nightcrawler fails a saving throw, it can choose to succeed instead.

Sunlight Hypersensitivity. While in sunlight, the Nightcrawler has disadvantage on attack rolls, as well as on Wisdom perception checks that rely on sight. While in Sunlight the Nightcrawler takes 40 radiant damage the the start of its turn. 

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

Magic Weapons. The Nightcrawler's natural weapon attacks are magical.

Spellcasting. The Nightcrawler is an 14th-level spellcaster. Its spellcasting ability is charisma (spell save DC 18, +10 to hit with spell attacks). The Nightcrawler has the following spells prepared:
Cantrips (at will): chill touch, toll the dead
1st level (4 slots): detect magic, magic missile
2nd level (3 slots): invisibility, see invisibility
3rd level (3 slots): dispel magic, haste
4th level (3 slots): banishment, hold monster, confusion
5th level (2 slots): cone of cold
6th level (1 slot): circle of death
7th level (1 slot): finger of death, plane shift

Multiattack: The Nightcrawler makes two attacks: one with its bite and one with its stinger.

Bite: Melee Weapon Attack: +15 to hit, reach 10 ft., one target. Hit: 35 (3d8 + 9) piercing damage (3d8) necortic damage. If the target is a Large or smaller creature, it must succeed on a DC 23 Dexterity saving throw or be swallowed by the Nightcrawler. A swallowed creature is Blinded and Restrained, it has total cover against attacks and other effects outside the Nightcrawler, and it takes 28 (6d8) necrotic damage at the start of each of the Nightcrawler's turns.

If the Nightcrawler 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 Nightcrawler. If the Nightcrawler dies, a swallowed creature is no longer Restrained by it and can escape from the corpse by using 20 feet of Movement, exiting prone.               

Any creature that dies after being swallowed by Nightcrawler becomes a Shadow undead the Nightcrawlers control 24 hours after death.  The Nightcrawler can choose to keep the shadow trapped within its stomach until a later time. The Nightcrawler can expel up to 8 Shadows from its mouth as an action. The Nightcrawler can hold up to 24 shadows within its stomach. 

Tail Stinger: Melee Weapon Attack: +15 to hit, reach 10 ft., one creature. Hit: 19 (3d6 + 9) piercing damage, and the target must make a DC 23 Constitution saving throw, taking 42 (12d6) necrotic damage on a failed save, or half as much damage on a successful one.

Frightful Presence. Each creature of the Nightcrawler's choice that is within 120 feet of the Nightcrawler and aware of it must succeed on a DC 16 Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a creature's saving throw is successful or the effect ends for it, the creature is immune to the Nightcrawler's Frightful Presence for the next 24 hours.

Summon Undead. 1/day the Nightcrawler can summon 2d8 shadows, 2d6 Specters or 2d4 Wraiths that are under the control of the Nightcrawler. These undead appear in a space the Nightcrawler can see within 60ft and attack on the Nightcrawler's turn. 

Can take 3 Legendary Actions, choosing from the options below. Only one legendary action can be used at a time, and only at the end of another creature's turn. Spent legendary actions are regained at the start of each turn.

Detect: The Nightcrawler makes a Wisdom (Perception) check.

Tail Attack: The Nightcrawler makes a tail Attack.

Burrow (Costs 2 Actions): The Nightcrawler can use its full movement to burrow 40ft

Shadowfell Summon (Cost 3 Actions): The Nightcrawler can summon 1d4 Wraiths, these creatures appear within 120ft of the Nightcrawler