Devil's Sight. Magical Darkness does not impede Zaviax's darkvision.

Fear Eater. Whenever a creature within 30 ft fails a saving throw against being frightened, Zaviax regains 15 hit points. 

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

Legendary Resistance (3/Day). If Zaviax fails a saving throw, he can choose to succeed instead.

Hellish Influence (Haunted Character Only). When (Haunted Character) experiences emotional distress (fear, grief, shame, insult, abandonment), they must succeed on a DC 16 Wisdom saving throw. On a failure, (Haunted Character) gains advantage on Strength-based attacks for 1 minute but takes 1d6 psychic damage at the start of each of their turns.

Predatory Bloodlust (Haunted Character Only). When (Haunted Character) sees a Small or Tiny creature with a CR lower than (Haunted Character's) level (such as goblins, kobolds, imps, small beasts), they must succeed on a DC 15 Wisdom saving throw. 

• On a Success: (Haunted Character) regains control.
• On a Failure: (Haunted Character) enters a Bloodthirsty Frenzy for 1 minute.

Bloodthirsty Frenzy: (Haunted Character) gains the following rules and effects: must move toward and attack the nearest qualifying creature, has advantage on Strength-based attack rolls, takes 1d6 psychic damage at the end of each turn, repeats the saving throw at the end of each of their turns, ending the frenzy on a success.

Innate Spellcasting. Zaviax is a 5-level spellcaster. Its spellcasting ability is Charisma (spell save DC 18,  +12 to hit with spell attacks). Zaviax has following spells prepared:
At Will: Command, Hellish Rebuke (3rd level), Minor Illusion, Thaumaturgy
3rd level (3 slots): Darkness, Suggestion, Crown of Madness, Hold Person, Fear

5th level (1 slot): Wall of Fire, Dominate Person

Multiattack. Zaviax makes three attacks: one with his Infernal Spear and two with his Barbed Tail. 

Infernal Spear. Melee Weapon Attack: +12 to hit, reach 10 ft., 1 target. Hit: 16 (2d6 + 9) piercing damage plus 14 (4d6) fire damage. 

Barbed Tail. Melee Weapon Attack: +10 to hit, reach 10 ft., 1 target. Hit: 12 (1d8 + 8) piercing damage plus 10 (3d6) poison damage. The target must succeed on a DC 18 Constitution saving throw or poisoned for 1 minute. The target may repeat the save at the end of each of its turns. 

Chains of Regret (Recharge 5–6). Each creature of Zaviax’s choice within 20 feet must succeed on a DC 18 Wisdom saving throw or be restrained by spectral chains for 1 minute.
A restrained creature takes 10 (3d6) psychic damage at the start of each its turns.
It may repeat the saving throw (DC 18 Wisdom saving throw) or attempt a DC 20 Strength saving throw to escape at the start of each of its turns.

Soul Burn (Mythic Form Only). Choose one frightened creature. It takes 14 (4d6) fire + 14 (4d6) psychic damage. If this kills the target, Zaviax regains 30 HP.

Legendary Actions (3 / Round)

Whisper (Costs 1 Action). A creature within 60 feet must succeed on a DC 17 Wisdom saving throw or have disadvantage all attacks until its next turn.

Move (Costs 1 Action). Zaviax moves up to half his flying speed without provoking opportunity attacks.

Tail Lash. (Costs 2 Actions). One Barbed Tail attack. 

Command the Weak. (Costs 3 Actions). A creature with HP lower than Zaviax's must succeed on a DC 18 Charisma save or be Charmed and forced to use its next turn to assist or defend Zaviax.

Activate when Zaviax reaches 0 HP for the first time.

He rises in a pillar of screaming flame.

Zaviax regains 150 HP. All enemies must make a DC 18 Wisdom save or become Frightened for 1 minute. 

New Mythic Trait

Avatar of Torment. At the end of each of his turns, one creature he can see takes 10 (3d6) psychic damage.