Agaroth the Vile

Legendary Fear Aura. Any creature hostile to Agaroth that starts its turn within 100 feet of him must make a DC 25 Wisdom or Charisma saving throw, unless he is incapacitated. On a failed save, the creature is frightened and paralyzed until the start of its next turn. If a creature's saving throw is successful, the creature is immune to Agaroth's Legendary Fear Aura for the next 12 hours.

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

Magic Weapons. Agaroth's weapon attacks are magical.

Innate Spellcasting. Agaroth's spellcasting ability is Charisma (spell save DC 24). Agaroth can innately cast the following spells, requiring no material components:

At will: detect magic, fireball

3/day each: hold monster, wall of fire

Spellcasting. Agaroth is an 18th-level spellcaster.  His spellcasting ability is Intelligence (spell save DC 22, +15 to hit with spell attacks).  Agaroth has the following wizard spells prepared:

Cantrips (at will): mage hand, prestidigitation, ray of frost

1st level (4 slots): detect magic, magic missile, shield, thunderwave

2nd level (3 slots): detect thoughts, invisibility, Melf's acid arrow, mirror image

3rd level (3 slots): animate dead, counterspell, dispel magic, fireball

4th level (3 slots): blight, dimension door

5th level (3 slots): cloudkill, scrying

6th level (3 slots): disintegrate, globe of invulnerability

7th level (3 slots): finger of death, plane shift

8th level (2 slots): dominate monster, power word stun

9th level (2 slots): power word kill 

Multiattack. Agaroth makes six attacks: one with his bite, two with his claw, two with his mace, and one with his 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 21 (6d6) 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: 21 (3d8 + 8) slashing damage.

Mace. Melee Weapon Attack: +14 to hit, reach 10 ft., one target. Hit: 24(5d6 + 8) bludgeoning damage plus 30 (10d6) fire damage.

Tail. Melee Weapon Attack: +14 to hit, reach 10 ft., one target. Hit: 28 (5d10 + 8) bludgeoning damage.

Fiery Inflict Wounds(2/day). This is a 9th level Inflict Wounds.  +14 to hit, touch, one target.  Hit:  11d10 necrotic damage, 11d10 fire damage.

Control Dead. If any creature has been killed or incapacitated by Agaroth's Fiery Inflict Wounds he can try and exert his control over their body.  The creature must make a DC 24 Charisma check with disadvantage or Agaroth gains control of their body.  This curse can be removed by a Remove Curse spell.  Or it will end when Agaroth is defeated.

Breath Weapons (Recharge 4-6). Agaroth uses one of the following breath weapons.

Hellfire Breath. Agaroth exhales fire in a 100-foot cone.  Each creature in that area must make a DC 25 Dexterity saving throw, taking 91 (26d6) fire damage on a failed save, or half that much damage on a successful one.  The hellfire ignores resistances granted by armor, race, spells, etc.

Weakening Breath. Agaroth exhales gas in a 100-foot cone.  Each creature in that area must succeed on a DC 25 Strength saving throw or have disadvantage on Strength-based attack rolls, Strength checks, and Strength saving throws for 1 minute.  A creature can repeat the saving throw at the end of each of its turns, ending the effect on a success.

Parry. Agaroth adds 4 to his AC against one melee attack that would hit it.  To do so, Agaroth must see the attacker and be wielding a melee weapon.

Detect. Agaroth makes a Wisdom (perception) check.

Bite Attack. Agaroth makes a bite attack.

Frightening Gaze (Costs 2 Actions). Agaroth fixes his gaze on one creature he can see within 30 feet of him.  The target must succeed on a DC 20 Wisdom saving throw against this magic or become frightened for 1 minute (10 Rounds).  The frightened target can repeat the saving throw at the end of each of its turns, ending the effect on a success.  If a target's saving throw is successful of the effect ends for it, the target is immune to Agaroth's gaze for the next 12 hours.

Vile Curse (Costs 3 Actions). Agaroth targets one creature he can see within 30 feet of him.  The target must succeed on a DC 17 Wisdom saving throw or be magically cursed.  Until the curse ends, the target has disadvantage on attack tolls and saving throws.  The target can repeat the saving throw at the end of each of its turns, ending the curse on a success.