Byoric Dread

Fear Eater. The Dread has advantage on attacks against enemies it has frightened, attacks against these targets do an additional 2d4 psychic damage. If a frightened creature is killed by the dread, it howls and recovers 3d8+4 hp.
Immutable Form. The slayer is immune to any spell or effect that would alter its form.

Brutal Attacks. Attacks made by the dread will crit on a 19 or 20.

Legendary Resistance (2/Day). If the dread fails a saving throw, it can choose to succeed instead.

Multiattack. The slayer makes 2 claw attacks, 1 bite attack, and a tail attack

Claw. Melee Weapon Attack: +13 to hit, reach 5 ft., one target. Hit: 14 (2d6 + 7) slashing damage. 

Bite. Melee Weapon Attack: +13 to hit, reach 5 ft., one target. Hit: 14 (2d6 + 7) piercing damage plus 11 (2d10) necrotic damage. A creature who is brought below 0 hp with this attack is devoured and is instantly killed. Any creatures who can see the dread must make a DC 17 wisdom saving throw or become frightened. Those who are frightened can repeat the save at the end of their turn.

Tail. Melee Weapon Attack: +13 to hit, reach 10 ft., one target. Hit: 21 (3d8 + 7) bludgeoning damage. 

Horrifying Screech (Recharge 6). The Dread exhales a piercing yell in a 60 foot cone. Each Creature must make a DC 20 charisma saving throw, taking 66 (12d10) thunder damage and becoming frightened on a failed save, or half damage on a successful one. Those who are frightened can repeat the save at the end of their turn.

Uncanny Targeting (Recharge 5-6). The dread moves up to its speed to another target, this movement does not trigger opportunity attacks.

Terrifying Gaze. Upon taking 25 or more damage in a single attack, the Dread can use its reaction to unleash a terrifying gaze at the one who dealt the damage. The target makes a DC 21 wisdom saving throw or becomes frightened for 10 rounds. Targets frightened by this gaze are stunned. Targets may repeat this save at the end of their turn.

Opportunity Attack. Opportunity attacks by the dread are made using it tail and so can be made at a range of 10 ft.

The slayer 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 creature's turn. The slayer regains spent legendary actions at the start of its turn.

Detect. The Dread makes a Wisdom (Perception) check with advantage.

Unholy Bile. The Dread spits bile at a target within 60 ft. The target must make a DC 15 Dexterity saving throw, taking 10 (4d4) Acid damage and having its speed halved during its next turn on a failed save, or half damage on a success.

Tail Whirl (2 actions). The Dread quickly swings its tail in a circle of 10 ft radius around itself. Anyone in the range must make a DC 17 Dexterity saving throw, taking 16 (2d8 + 7) bludgeoning damage and knocked prone on a failed save, or half damage on a success.

Terrible Charge (3 actions). The dread can move its full speed, opportunity attacks against it deal half damage and are made at disadvantage. It makes a claw attack against any enemy it comes within 5ft of.

Enter the description for how mythic actions work for your monster here.

Mythic Action Name (Costs # Actions). Enter the description for your mythic action.