Cæruleumbra (Azure Shadow)

Discorporation. When Cæruleumbra drops to 0 hit points or dies, his body is destroyed but his essence travels back to his domain in the Nine Hells, and he is unable to take physical form for a time.

Spellcasting.  Cæruleumbra is a 20-level spellcaster. His spellcasting ability is charisma (spell save DC 23, +15 to hit with spell attacks). Cæruleumbra has following spells prepared: 

Cantrips (at will): Acid Splash, Fire Bolt, Eldritch Blast, Chill Touch, Poison Spray

1st level (5 slots): Burning Hands, Chromatic Orb, False life, Tasha's Hideous Laughter

2nd level (4 slots): Blindness/deafness, Crown of Madness, Flaming sphere, Scorching Ray

3rd level (4 slots): Animate Dead, Lightning Bolt, Fireball

4th level (4 slots): Banishment, Evard's Black Tentacles, Phantasmal Killer, Wall of Fire

5th level (4 slots): Cone of Cold, Modify Memory, Wall of Force

6th level (3 slots): Chain Lightning, Circle of Death, Flesh to Stone

7th level (3 slots): Delayed Blast Fireball, Finger of Death, Teleport

8th level (3 slots): Control Weather, Dominate Monster

9th level (2 slots): Power Word Kill, Weird

Legendary Resistance (5/Day). If Cæruleumbra fails a saving throw, he can choose to succeed instead.

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

Magic Weapons. Cæruleumbra’s weapon attacks are magical.

Regeneration. Cæruleumbra regains 30 hit points at the start of his turn. If he takes radiant damage, this trait doesn’t function at the start of his next turn. Cæruleumbra dies only if he starts his turn with 0 hit points and doesn’t regenerate.

Multiattack. Cæruleumbra can use his Frightful Presence. He then makes three attacks: two with his claws and one with his ranseur. He can substitute Tail or Bite for one of these attacks.

Claw. Melee Weapon Attack: +19 to hit, reach 15 ft., one target. Hit: 24 (4d6 + 10) slashing damage.

Tail. Melee Weapon Attack: +19 to hit, reach 25 ft., one target. Hit: 28 (4d8 + 10) bludgeoning damage.

Bite. Melee Weapon Attack: +19 to hit, reach 20 ft., one target. Hit: 32 (4d10 + 10) piercing damage plus 14 (4d6) cold damage.

Ranseur. Melee Weapon Attack: +16 to hit, reach 10 ft., one target. Hit: 17 (2d8 + 8) slashing damage, or 19 (2d10 + 8) slashing damage if used with two hands, plus 36 (8d8) cold damage.

Frightful Presence. Each creature of Cæruleumbra's choice within 120 feet of it and aware of it must succeed on a DC 26 Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, with disadvantage if Cæruleumbra is within line of sight, 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 Cæruleumbra's Frightful Presence for the next 24 hours.

Swallow. Cæruleumbra makes one bite attack against a Large or smaller creature it is grappling. If the attack hits, the target takes the bite's damage, the target is swallowed, and the grapple ends. While swallowed, the creature is blinded and restrained, it has total cover against attacks and other effects outside Cæruleumbra, and it takes 56 (16d6) acid damage at the start of each of Cæruleumbra's turns.

If Cæruleumbra takes 60 damage or more on a single turn from a creature inside it, Cæruleumbra must succeed on a DC 20 Constitution saving throw at the end of that turn or regurgitate all swallowed creatures, which fall prone in a space within 10 feet of Cæruleumbra. If Cæruleumbra dies, a swallowed creature is no longer restrained by it and can escape from the corpse by using 30 feet of movement, exiting prone.

Summon Devils (1/day). Cæruleumbra summons any amount of devils as long as they have a combined challenge rating of 20 or less. Alternatively, he can use this ability to summon 2 Pit Fiends. 

Teleport. Zariel magically teleports, along with any equipment she is wearing and carrying, up to 120 feet to an unoccupied space she can see.

Cæruleumbra can take 5 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature’s turn. Unhallowed One regains spent legendary actions at the start of his turn.

Attack. Cæruleumbra makes one claw attack or tail attack.

Move. Cæruleumbra moves up to half its speed.

Chomp (Costs 2 Actions). Cæruleumbra makes one bite attack or uses its Swallow.

Breath of Baator (Costs 2 Actions). Cæruleumbra breathes an icy blast in a 90-foot cone. Each creature in that area must make a DC 27 Dexterity saving throw, taking 72 (16d8) cold damage on a failed save, or half as much damage on a successful one.