Astaroth, King of Rot

Corrupted Weapons. Astaroth's weapon attacks are magical. When Astaroth hits with any weapon, the weapon deals an extra 27 6d8 necrotic damage (included in the attack).

Innate Spellcasting. Astaroth's spellcasting ability is Charisma (spell save DC 27). It can innately cast the following spells, requiring no material components:

At Will: cause fear, command, silvery barbs, fireball

5/day each: counterspell, destructive wave, circle of death, finger of death

3/day each: power word pain, power word stun

1/day each: paradox, psychic scream

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

Regeneration. Astaroth regains 20 hit points at the start of their turn.

Legendary Resistance (5/Day). If Astaroth fails a saving throw, he can choose to succeed instead.

Multiattack. Astaroth makes two attacks with its Withering Scythe.

Withering Scythe. Melee Weapon Attack: +19 to hit, reach 10 ft., one target. Hit: 27 (3d10 + 10) slashing damage plus 27 (6d8) necrotic damage.

Bale Fire (Recharge 5–6). Astaroth projects black fire from his hand in a 100-foot cone. Each creature in that area must make a DC 25  Dexterity saving throw, taking 41 (13d6) fire damage and 41 (13d6) necrotic damage on a failed save, or half as much damage on a successful one.

Teleport. Astaroth teleports, along with any equipment he is wearing or carrying, up to 120 feet to an unoccupied space he can see

Innate Spellcasting. Astaroth can use their reaction to cast silvery barbs or counterspell 

Astaroth can take 3 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. Astaroth regains spent legendary actions at the start of its turn.

Teleport. Astaroth magically teleports, along with any equipment it is wearing or carrying, up to 120 feet to an unoccupied space it can see.

Attack. Astaroth makes one attack with his Withering Scythe.

Eminence of Rot (Costs 2 Actions). Astaroth radiates an aura of Necrotic Energy. Each creature within 20 feet of Astaroth must succeed on a DC 25 Constitution saving throw or take 44 (8d10) necrotic damage

Once Astaroth reaches less than half of its max hit points, he can use the options below as legendary actions.

Baleful Aura (Costs 2 Actions). Horrible energies temporarily surround Astaroth, and it gains 40 temporary hit points until the start of its next turn. Until all these temporary hit points are gone, any creature that touches Astaroth or hits it with a melee attack takes 26 (4d12) necrotic damage.

Bale Fire Spears (Costs 3 Actions). Astaroth creates four spears of malignant energy. Each spear hits a creature of the Astaroth's choice it can see within 120 feet of it, dealing 22 (4d10) necrotic damage and 22 (4d10) fire damage to its target, then disappears.