Legendary Resistance (3/Day). If Abithriax fails a saving throw, he can choose to succeed instead. When he does, attack rolls made against him have advantage until the start of his next turn.

Multiattack. Abithriax makes 2 Rend attacks and 1 Bite attack. He can replace the bite attack with a cast of Scorching Ray.

Rend. Melee Attack Roll: +10, reach 5 ft. Hit: 11 (1d10 + 6) Slashing damage plus 5 (2d4) Fire damage.

Bite. Melee Attack Roll; +10, reach 10 ft. Hit: 17 (2d10 + 6) Piercing damage plus 7 (2d6) fire damage and the target is set on fire for 1 minute or until they or another creature uses an action to extinguish the flames. A creature on fire takes 7 (2d6) fire damage at the start of their turn.

Fire Breath (Recharge 5–6);. Dexterity Saving Throw: DC 17, each creature in a 30-foot Cone. Failure: 56 (16d6) Fire damage. Success: Half damage.

In addition, the fire remains in the area until the start of Abithriax's next turn. Any creature who enters the area for the first time on a turn or who ends their turn there takes 7 (2d6) fire damage.

Spellcasting. Abithriax casts one of the following spells, requiring no Material components and using Charisma as the spellcasting ability (spell save DC 17, +9; to hit with spell attacks):

At Will: Command (level 2 version), Detect Magic, Scorching Ray

Frightful Prescence. Wisdom Saving Throw: DC 17, 1 target, 60 ft. Failure: Frightened for 1 minute (save end of turn).

Detect. Abithriax makes a Wisdom (Perception) check.

Ignite the Insolent. When Abithriax is hit with an attack, or affected by a spell, the attacker must succeed on a DC 17 Dexterity Saving Throw. On a failure, the target is set on fire for 1 minute or until they or another creature use an action to extinguish the flames. A creature on fire takes 7 (2d6) fire damage at the start of their turn.

Villain Action 1: Cinderfall  A 30 ft. radius cloud of cinder and smoke surrounds Abithriax until the end of his next turn. That area is heaving obscured for creatures other than Abithriax. Creatures who need to breathe and start their turn inside the area must succeed on a DC 17 Constitution Saving Throw or become Poisoned for 1 minute (save end of turn).

Villin Action 2: Come, Little Mice. Up to 3 creatures within 60 ft. of Abithriax must succeed on a DC 17 Wisdom Saving Throw or use their reaction (if available) to move up to their speed towards him.

Villain Action 3: Supernova.  Abithriax unleashes a massive burst of light and heat. All creatures within 120 ft. must make a DC 18 Contitution Saving Throw. On a Failure, they take 21 (6d6) fire damage plus 21 (6d6) radiant damage and become Blinded. On a Success, they take half as much damage and are not blinded. The blindness lasts until the creature completes a Long Rest, is cured by a successful DC 18 Medicine Check or a spell / magical effect.