Croth the Devourer

Legendary Resistances (5/day). If Croth fails a saving throw he can choose to succeed instead.

Magic Resistance: Croth has advantage on saving throws against spells and other magical effects.

Seething Pain: All creature's that start their turn within 30ft of Croth must succeed on a DC 20 Constitution saving throw or become racked with severe pain for 1minute. Whilst racked with pain creatures have disadvantage on all attack rolls and saving throws and their movement speed is reduced to 10ft for the duration. If a creature attempt to cast a spell whilst racked with pain, they must first succeed on a constitution saving throw or the casting fails, and the spell is wasted. A creature repeats the save at the end of each of their turns, ending the effect on itself on a success. On a successful save creatures become immune to seething pain ability for the next 24 hours.

Multiattack. Croth makes 2 Claw attacks followed by either Devour entity or Horrifying whispers.

Claw. Melee Weapon Attack. 15ft reach; +18 to hit, one target: Hit: 3d10+10 slashing damage plus 4d6 necrotic damage.

Devour Entity (Recharge 5 or 6). Croth chooses one creature he can see within 30ft of him, the creature must succeed a DC 21 Intellect saving throw or be teleported inside Croth's chest taking 8d10 necrotic damage. Croth regains hit points equal to half of the necrotic damage dealt, the creature is then teleported back to its previous position. If the target is reduced to 0 hit points in this way it is completely devoured, and its soul is absorbed into the Abyss.

Horrifying Whispers. Croth peers into the mind of one creature it can see within 60ft. The creature must make a DC 21 Wisdom saving throw, taking 10d6 psychic damage and on a failed save and half as much damage on a success. If a creature fails this save by 5 or more it is paralyzed in fear until the start of its next turn (creatures immune to fear cannot be paralyzed by this effect).

Shadow Walk: Whilst within dim light or darkness, as a bonus action, Croth can can magically teleport up-to 120ft to an unoccupied space he can see that is also in dim light or darkness.

Croth the Devourer 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 creatures turn. Croth regains spent legendary actions at the start of its turn.

Claw (1 action). Croth makes one Claw attack.

Dark Hammer (2 actions). Croth summons a massive Black Hammer from the Abyss and makes one weapon attack with it. The attack is a Melee Weapon Attack. 20ft reach; +18 to hit, one target. Hit: 6d12 +10 bludgeoning damage plus 4d6 necrotic damage. This weapon is considered magical and critically hits on an 18-20 roll. 

Summon Demon (2 actions). Croth summons a Demon to an unoccupied space he can see within 90ft. The Demon shares Croth's initiative and takes its turn immediately after his. The summoned Demon is chosen at random from the list below (roll 1d6).

1. Barlgura, 2. Chasme, 3. Vrock, 4. Shoosuva, 5. Hezrou, 6. Glabrezu.