Magic Skin. The zathroth's weapon attacks are magical.

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

Legendary Resistance (3/Day). If the zathroth fails a saving throw, it can choose to succeed instead.

Infernal Presence. Any creature hostile to the zathroth withing 100 feet, loses immunity to necrotic and fire damage, and cannot regain it.

Infernal Corruption. Any no chaotic evil creature that starts its turn withing 100 feet of the zathroth, must make a DC 18 charisma saving throw, if failed change the alignment to chaotic evil and taking 55 (10d10) necrotic damage.

Infernal Subjugation. Any chaotic evil creature hostile to the zathroth that starts its turn withing 100 feet of the zathroth, must make a DC 25 charisma saving throw. On a failed save, the creature is incapacitated until the end of the turn.

Infernal Aura. At the start of each of the zathroth's turns, each creature within 35 feet of it takes 10 (3d6) fire damage plus 15 (6d4) necrotic damage, and flammable objects in the aura that aren't being worn or carried ignite. A creature that touches the zathroth or hits it with a melee attack while within 35 feet of it takes 10 (3d6) fire damage plus 15 (6d4) necrotic damage.

Innate Spellcasting. The zathroth's spellcasting ability is Charisma (spell save DC 25). The zathroth can innately cast the following spells, requiring no material components:

At will: fireball, wall of fire.

4/day: flame strike.

2/day: fire storm.

1/day: meteor swarm.

Multiattack. The zathroth can use a wings, tail or bite and makes two attacks with its claws.

Bite. Melee Weapon Attack: +18 to hit, reach 10 ft., one target. Hit: 21 (2d10 + 10) piercing damage, plus 10 (4d4) fire damage, plus 10 (4d4) necrotic damage.

Claw. Melee Weapon Attack: +18 to hit, reach 15 ft., one target. Hit: 19 (2d8 + 10) slashing damage, plus 5 (2d4) fire damage.

Tail. Melee Weapon Attack: +16 to hit, reach 60 ft., one target. Hit: 17 (2d6 + 10) bludgeoning damage plus 5 (2d4) fire damage, if the target is size huge or less, must succeed on a DC 30 Strength saving throw or be pulled up to 30 feet toward the zathroth.

Wings. The zathroth beats its wings, creating sharp winds in a 45-foot cone. Each creature in that area must succeed on a DC 25 Dexterity saving throw or take 15 (2d4 + 10) slashing damage and be knocked prone.

Infernal Breath (Recharge 5-6). The zathroth exhales a black fire in a 90-foot cone. Each creature in that area, taking 39 (6d12) fire damage and must make a DC 25 constitution saving throw, if failed taking an extra 39 (6d12) necrotic damage, and become poisoned. While poisoned in this way, the target can't regain hit points, and it takes 20 (8d4) fire damage, plus 20 (8d4) necrotic 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.

The zathroth can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only a the end of another creature's turn. The zathroth regains spent legendary actions at the stard of its turn.

Teleport. The zathroth magically teleports, along with any equipment it is wearing or carrying, up to 200 feet to an unoccupied space it can see.

Cast a Spell (Costs 2 Actions). The zathroth casts a spell from its list of innate spells.

Infernal Heart (Costs 3 Actions). The zathroth inspires a demonic ferocity in all surrounding demon allies. All demon allies withing 100 feet of the zathroth deals an extra 14 (4d6) damage when it hits with a weapon attack (included in the attacks), until the end of the next turn of zathroth.

Infernal Portal (Costs 3 Actions). The zathroth opens an infernal portal, in an unoccupied space up to 100 feet away from him, that he can see. Roll a d20 and summon a demon there, depending on the result. (20: Balor, 19-18: Marith, 17-15: Nalfeshmee, 14-11: Glabrezy, 10-1: noting).