Working on one of the lesser known Demon Lords for my Out of the Abyss game. I used Titivilus to scale off of, and introduced ideas from the 3.5 prestige class "Dread Witch."
The goal is an archfiend that is feared by his own kind, and can control any room that he is in. As a one-trick pony, he should be easy enough to take down with preparation. Yet, it should be one of the most fearsome of encounters if caught unaware.
His Marked for Fear ability should be taken largely as one for flavor. If you are his target, he can make you feel haunted by the fear of something pursuing you. It intentionally uses Fear as a spell so it can be dispelled if the target catches on to the source of it's fear.

Anyway, please comment on appropriate verbiage, challenge rating, mistakes, or any ideas you think could enhance his build.

Alvarez, known as the Purging Duke, is the demon lord of Torture and Inquisition. He searches the ranks of the tanar'ri incessantly for signs of the taint of Law. As a mere petitioner, he outwitted the nalfeshnee of Woe who judged him, and served as a maggot-loving chasme and then a powerful glabrezu assassin (for 3,000 years) before gaining a demonic form closely resembling his living one: a human man with oily blue-black hair. His clothing is immaculate, but a light of insanity burns in his eyes.

Alvarez
Medium Fiend (demon), chaotic evil
161 (17d8HD+85)
Str 17
Dex 19
Con 21
Int 19
Wis 23
Cha 24
Saves: Wisdom, Charisma, Strength

Proficiencies:
Intimidation
Insight
Religion
Athletics

Damage Resistances: cold, fire, lightning
Damage immunities: Poison. Bludgeoning, Piercing, and slashing that is nonmagical
Condition Immunities: Charmed, exhaustion, frightened, poisoned
Senses: truesight 120 ft., passive perception 23
Languages: all, telepathy 120 ft.
Challenge 18

Innate Spellcasting. Alvarez's innate spellcasting ability is Charisma (spell save DC 20). He can innately cast the following spells, requiring no material components:
At Will: Protection from Evil and Good, Hunter's Mark, Command, Crown of Madness, Misty Step, Fear, Bestow Curse, Inflict Wounds (As a 5th-level spell), Magic Circle
3/day: Zone of Truth, Dispel Magic, Counterspell, Hold Monster, Scrying
1/day: Dominate Monster, Imprisonment, Teleport, Modify Memory, Haste

Legendary resistance (3/Day) If Alvarez fails a saving throw, he can choose to succeed instead.

Magic Resistance. Alvarez has advantage on saving throws against spell and other magic effects.

Magic Weapon. Alvarez's weapon attacks are magical.

Mastery of Terror - Any creature that is immune to being frightened instead only has advantage on the saving throw to resist any fear effects attempted by Alvarez. Additionally, any creature that has the frightened status condition caused by Alvarez also has disadvantage to any saving throw Alvarez requires the creature to make.

Marked for Fear - If a creature is currently under Alvarez's Hunter's Mark spell, Alvarez may cast the Fear spell upon the creature, regardless of distance or line of sight, as if the spell descriptor said "single creature."

Actions:

Multiattack. Alvarez makes one sword attack and uses his Frightful Word once.

Irresistible Truth Whip. Melee Weapon Attack: +9 to hit, reach 10 ft., one target. Hit: 8 (2d4+4) slashing damage, plus 16 (3d10) necrotic damage. If the target is a creature, its hit point maximum is reduced by an amount equal to half the necrotic damage it takes.

Frightful Word. Alvarez targets one creature he can see within 30 feet of him. The target must succeed on a DC: 20 Wisdom saving throw or become frightened for 1 minute. While frightened in this way, the target must take the Dash action and move away from Alvarez by the safest available route on each of its turns, unless there is nowhere to move, in which case it needn't take the Dash action. The target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.

Legendary Actions:
Alvarez 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. Alvarez regains spent legendary actions at the start of his turn.
- Assault. Alvarez attacks once with Irresistible Truth or uses his Frightful Word
- Alvarez casts Inflict Wounds
- Paralyzing Fear (Costs 2 Actions) One creature who is afraid of Alvarez that he can see is paralyzed until the end of Alvarez's next turn.