Azelthar, Ruler of the Shadow Dragons, Progenitor of the Shadowfell

Amphibious: Azelthar can breathe air and water.

Aberrant Ground.  The ground in a 10-foot radius around the monster is difficult terrain. Each creature that starts its turn in that area must succeed on a DC 24 Strength saving throw or have its speed reduced to 0 until the start of its next turn.

Aggressive. As a bonus action, the monster can move up to its speed toward a hostile creature that it can see.

Air Form. The monster can enter a hostile creature’s space and stop there. It can move through a space as narrow as 1 inch wide without squeezing.

Avoidance. The monster will take no damage from succeeding on spell save DC rolls and half damage on failures.

Axiomatic Mind. The monster can’t be compelled to act in a manner contrary to its nature or its instructions.

Magic Resistance

Shadow Veil: Can hide in Shadows, becoming impossible to view

Damage Absorption. Whenever the monster is subjected to necrotic or shadow damage, it takes no damage and instead regains a number of hit points equal to the damage dealt.

Displacement. The monster projects a magical illusion that makes it appear to be standing near its actual location, causing attack rolls against it to have disadvantage. If it is hit by an attack, this trait is disrupted until the end of its next turn. This trait is also disrupted while the monster is incapacitated or has a speed of 0.

Legendary Resistance: 5

Unusual Nature

Automatic Reaction: creature gets two reactions.

Limited Magic Immunity. The creature can't be affected or detected by spells of 6th level or lower unless it wishes to be.

Shadows Growing:  Upon being killed, Azelthar will gain 500 hit points, all expended legendary actions,  and can use mythic actions for one hour.

Spellcasting. Azelthar is a 27-level spellcaster. Its spellcasting ability is Charisma (spell save DC 30, +21 to hit with spell attacks). Azelthar has following Sorcerer and Warlock spells prepared:

1st level (5 slots) 2nd level (5 slots) 3rd level (5 slots) 4th level (5 slots) 5th level              (4 slots) 6th level (4 slots) 7th level (3 slots) 8th level (3 slots) 9th level (3 slots)
Absorb Elements, Disguise Self,  Unseen Servant, Hex, See Invisibility, Wither and Bloom Counterspell, Dispell Magic, Fear, Vampyric Touch, Shadow Shadowspawn, Banishment, Shadow of Moil, Eyebite, Soul Cage, Finger of Death, Power Word: Pain, Glibness, Power Word Kill, Conjure Shadow Shadow, Cloudkill, Incendiary Cloud, Anti Magic Field

Frightful Presence: Each creature of the dragon's choice that is within 120 feet of the dragon and aware of its presence 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 dragon's Frightful Presence for the next 24 hours

Shadow Claw. Claws begin to rise from the shadow of the target Melee Weapon Attack: +10 to hit, reach 120 ft., 1 target. Hit: 56 (9d10 + 10) Necrotic damage + 56 (9d10 + 10) Shadow damage. 

Bite: The dragon bites down on the target, Melee Weapon Attack: +10 to hit, reach 20 ft., 1 target. Hit: 56 (9d10 + 10) Necrotic damage + 56 (9d10 + 10) Piercing damage.

Claw: The dragon strikes with a claw wreathed in darkness, pulling the light out of the air. Melee Weapon Attack: +10 to hit, reach 20 ft., 1 target. Hit: 56 (9d10 + 10) Necrotic damage + 56 (9d10 + 10) Slashing damage.

Shadow Forge: The dragon channels its power through the shadows manifesting them as physical creations, creating pillars slamming from the ground. DC 28 Dex, 1-3 Targets. On hit, targets take 79 shadow damage and 58 bludgeoning damage and become prone, or half as much on a succesful one. Evasion does not work.

Multiattack: The dragon uses frightful presence, then one bite attack, two claw attacks, and 1 Shadow Forge.

CastStrike: After casting a spell of 5th level or lower, creature makes one claw or bite attack.

Shadows Embrace: Traps one creature in a pocket dimension of shadows with 5 Shadow Dragon Essence's, mirroring the effect of Eternal Night. Can only escape after killing all 5 Shadow Dragon Essence's, succeeding a DC 25 Insight Check, or using an attack that deals 50 or more radiant damage.

4 Actions, Recharge at end of creature turn.

Shadows Strike (Costs 2 Actions): The dragon slams its claws on the ground. two creatures in a 240 ft radius must make a DC 23 Dexterity saving throw or take and a level of exhaustion, or half as much on a succesful one and no exhaustion.

Bite (Costs 1 Action): Dragon makes a bite attack

Eternal Night (Costs 2 Actions). Releases an aura of Darkness. Until the effect ends, the area in a 800 ft radius is cloacked in pitch darkness. All creatures that are not of shadow take -11 on attack rolls and saving throws. As well as this, shadow creatures are classed as having full cover and get advantage on attack rolls.

Shadows Wrath (Costs 2 actions): Summons the full extent of nearby shadows. All creatures on a 120 ft radius are affected. +10 to hit, deals 69 shadow damage or 124 shadow damage if under Eternal Night.