The great despoiler. Allarak curses anyone he attacks, dealing 1d10 damage of the type that the target is most vulnerable to. If possible, deal acid damage. In addition, the target of one of Allarak's attacks cannot benefit from having a resistance to the damage type that attack deals. Allarak can also survive anywhere, no matter the conditions.

Multiattack. Allarak makes 2 Claw and 2 Bite actions. Any of these can be replaced with acidic blast attacks or whip-tail attacks.

Claw. Melee Weapon Attack: +10 to hit, reach 10 ft., 1 target. Hit: 46 (10d8 + 6) slashing damage. 

Bite. Melee Weapon Attack: +10 to hit, reach 5 ft., 2 targets. Hit: 52 (7d12 + 6) piercing damage.

Acidic blast. RangedWeapon Attack: +10 to hit, range 200/400 ft., all targets in a 30 foot cone. Hit: 15 (10d10 + 2) acid damage.

Whip-tail. RangedWeapon Attack: +10 to hit, range 10/20 ft., 2 targets. Hit: 10 (5d8) bludgeoning damage.

Annihilating sweep. When Allarak is in close combat and gets attacked from behind, he can use a whip-tail attack against the attacker. This is a normal attack with the following added:

The target, and all creatures within 20 feet horizontally sideways from the target, get hit by the attack rather than just 2.

All creatures hit are knocked prone. Creatures that are huge or larger are not affected by this but still take damage.

Creatures who are small take 3d4 bludgeoning damage on top of anything else. Tiny creatures die automatically if hit.

Allarak cannot attack with a melee attack in the following round.

Allarak can make 3 legendary actions, which recharge when allarak starts his turn.

Roar of ancient fury (Costs 2 Actions). Allarak roars, forcing every creature within 100 feet to take a DC 15 strength check or be knocked prone and stunned for 20 minutes. Creatures not visible to Allarak are not knocked prone but are still stunned. A stunned creature repeats the save at the start of each of its turns, ending the effect on itself on a success. If a target saves against this action, they are immune to allaraks roar of ancient fury for another 24 hours.

Acid blast (Costs 1 bonus Action). Allarak makes an acidic blast attack.

Domination. (Costs 3 Actions). Allarak attempts to psychically dominate one creature he can see within 100 feet.That creature must make a DC 13 CON save or be dominated. He can command that creature for as long as he is alive, or until the dominated creature is knocked unconscious or killed . This action takes 3 long rests to recharge, and Allarak can only have 2 creatures under his control at a time. While a creature is dominated by Allarak, they are charmed by him and cannot remember anything about their real life or themselves.

Allarak can make 1 Mythic action per 5 long rests.

Gaze of Vecna (Costs 1 Actions). Allarak glares at all creatures in a 30 foot cone. These creatures must make a DC 14 CON save or be petrified for 10 rounds.