Fear Aura. Any creature hostile to the pit fiend that starts its turn within 20 feet of Zorat must make a DC 21 Wisdom saving throw, unless the pit fiend is incapacitated. On a failed save, the creature is Frightened until the start of its next turn. If a creature's saving throw is successful, the creature is immune to Zorat's Fear Aura for the next 24 hours.

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

Magic Weapons. Zorat's weapon attacks are magical.

Innate Spellcasting. Zorat's spellcasting ability is Charisma (spell save DC 21). The pit fiend can innately cast the following spells, requiring no material components:

At will: detect magic, fireball

3/day each: hold monster, wall of fire

Multiattack. Zorat makes four attacks: one with its bite, one with its claw, one with its mace, and one with its tail.

Bite. Melee Weapon Attack: +14 to hit, reach 5 ft., one target. Hit: 22 (4d6 + 8) piercing damage. The target must succeed on a DC 21 Constitution saving throw or become poisoned. While poisoned in this way, the target can't regain hit points, and it takes 42 (6d12) poison damage at the start of each of its turns. The poisoned target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.

Claw. Melee Weapon Attack: +14 to hit, reach 10 ft., one target. Hit: 17 (2d8 + 8) slashing damage.

Mace. Melee Weapon Attack: +14 to hit, reach 10 ft., one target. Hit: 15 (2d6 + 8) bludgeoning damage plus 21 (6d6) fire damage.

Tail. Melee Weapon Attack: +14 to hit, reach 10 ft., one target. Hit: 24 (3d10 + 8) bludgeoning damage.

Zorat the Plaguebearer can take 1 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature's turn. The dragon regains spent legendary actions at the start of its turn.

Virulent Disease (Cost 1 actions).

Zorat unleashes a virulent disease on a creature that he can see within range. The target must make a Constitution saving throw. On a failed save, it takes 14d6 necrotic damage, or half as much damage on a successful save. The damage can't reduce the target's hit points below 1. If the target fails the saving throw, its hit point maximum is reduced for 1 hour by an amount equal to the necrotic damage it took. Any effect that removes a disease allows a creature's hit point maximum to return to normal before that time passes.

Demonic Contagion (Cost 1 Actions).

Zorat's touch inflicts disease. Make a melee spell attack against a creature within your reach. On a hit, the target is poisoned.

At the end of each of the poisoned target’s turns, the target must make a Constitution (DC21)  saving throw. If the target succeeds on three of these saves, it is no longer poisoned, and the spell ends. If the target fails three of these saves, the target is no longer poisoned, but choose one of the diseases below. The target is subjected to the chosen disease for the spell’s duration.

Since this spell induces a natural disease in its target, any effect that removes a disease or otherwise ameliorates a disease’s effects apply to it.

Blinding Sickness. Pain grips the creature’s mind, and its eyes turn milky white. The creature has disadvantage on Wisdom checks and Wisdom saving throws and is blinded.

Filth Fever. A raging fever sweeps through the creature’s body. The creature has disadvantage on Strength checks, Strength saving throws, and attack rolls that use Strength.

Flesh Rot. The creature’s flesh decays. The creature has disadvantage on Charisma checks and vulnerability to all damage.

Mind-fire. The creature’s mind becomes feverish. The creature has disadvantage on Intelligence checks and Intelligence saving throws, and the creature behaves as if under the effects of the confusion spell during combat.

Seizure. The creature is overcome with shaking. The creature has disadvantage on Dexterity checks, Dexterity saving throws, and attack rolls that use Dexterity.

Slimy Doom. The creature begins to bleed uncontrollably. The creature has disadvantage on Constitution checks and Constitution saving throws. In addition, whenever the creature takes damage, it is stunned until the end of its next turn.