Cororthad

Unusual Nature. The Cororthad doesn't require air, food, drink, or sleep.

Ring Bearer. The Cororthad can sense the presence of its ring within 100 feet.

Shadow Call (Mythic Trait; Recharges after Short or Long Rest). If the Cororthad is being fought at night or in a dark place regardless of artificial light sources then if the Cororthad is reduced to 0 hitpoints, it is doesn't die or fall unconscious. Instead, it regains 123 hit points and emits a high-pitched scream. All other creatures within 30 feet must make a DC 15 Wisdom saving throw. On a failed save, the creature will take 2d6+5 psychic damage. The Cororthad will gain Resistance to the following types of damage: bludgeoning, piercing, lightning, and thunder from nonmagical attacks.

Multiattack. The Cororthad may attack twice with its longsword.

Longsword. Melee Weapon Attack: +8 to hit, reach 5 ft., One target. Hit: 12 (1d8 + 8) [slashing] damage plus 3 (1d6) [necrotic] damage. 

Longsword (2-handed). Melee Weapon Attack: +8 to hit, reach 5 ft., One target. Hit: 13 (1d10 + 8) [slashing] damage plus 3 (1d6) [necrotic] damage. 

Curuwath Knife. Melee Weapon Attack: +8 to hit, reach 5 ft., One target. Hit: 10 (1d4 + 8) [slashing] damage plus 3 (1d6) [necrotic] damage. If a creature is hit by this knife, it must succeed a DC 13 Constitution saving throw or be paralyzed until the end of the creature's next turn.

Piercing Wail. The Cororthad emits a high-pitched scream. All other creatures within 30 feet must make a DC 13 Wisdom saving throw. On a failed save, a creature is frightened and stunned until the end of the Cororthad’s next turn and if it’s concentrating on a spell, it loses its concentration.

Shrieking Slash. Melee Weapon Attack: +8 to hit, reach 5 ft., One target. Hit: 12 (1d8 + 8) [necrotic] damage. The Cororthad will attack and shriek loudly at a creature of its choice within range. The creature must make a DC 12 Constitution saving throw. On a failed save the creature will become blinded and deafened until the end of the Cororthad's next turn.

The Cororthad can take 2 legendary actions, choosing from the options below. Only one legendary option can be used at a time and only at the end of another creature's turn. The Cororthad regains spent legendary actions at the start of its turn.

Retreat (Costs 1 Action). The Cororthad may retreat from battle by disappearing into a spiritual form without provoking attacks of opportunity.

Wailing Strike (Costs 1 Action). Melee Weapon Attack: +8 to hit, reach 5 ft., One target. Hit: 12 (1d8 + 8) [slashing] damage. After attacking, the Cororthad will wail. This will add 1d10 damage to the first attack it makes after this. The effects of this attack may only be counted once for a single attack.

If the Cororthad's Mythic Trait is active then it can use the option below as a legendary action

Shadow Slash (Costs 1 Action). Melee Weapon Attack: +8 to hit, reach 5 ft., One target. Hit: 12 (1d8 + 8) [slashing] damage plus 3 (1d6) [necrotic] damage. The Cororthad will disappear into a spiritual form and reappear at a point of its choice. Any creature currently occupying the chosen space must make a DC 15 Dexterity saving throw. On a successful save, the creature will move to a space 5ft away. On a failed save, the creature is knocked prone and must take 1d6 bludgeoning damage. The prone creature must succeed a Strength (Athletics) or Dexterity (Acrobatics) check to escape from beneath the Cororthad's feet. Upon reappearing, the Cororthad will attack all creatures within a 5ft radius.