Arinellious, Archdevil of Deceit

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

Legendary Resistance (3/day): If Arinellious fails a saving throw, it can choose to succeed instead.

Magic Weapons: Arinellious's weapon attacks are magical.

Spellcasting: Arinellious is a 17th level spellcaster. His spellcasting ability is Charisma (Spell save DC 23). He can cast the following spells requiring no material components.

-At will: Disguise Self, Detect Magic, Detect Thoughts, Minor Illusion, Invisibility, Firebolt, Burning Hands

-2/day each: Greater Invisibility, Blur, Mirror Image, Shadow Blade, Fear, Charm Person, Major Image

-1/day each: Modify Memory, Dominate Person, Dominate Monster, Mislead

Enhanced Spellcasting: Arinellious has the ability to concentrate on 2 spells at the same time if they are of the Illusion or Enchantment schools of magic. He must make concentration saving throws for each spell if he is concentrating on 2 at the same time.

Illusory Appearance: Arinellious can create a tangible illusion over himself once per day that lasts as long as he desires. While this illusion is over him he takes the appearance of a large Pit Fiend and has extra attack capabilities. A creature can discern if his appearance is an illusion with a successful DC 20 Intelligence (Investigation) or DC 23 Wisdom (Perception) check. Arinellious uses this form to hide his smaller form in order to intimidate and trick people into deals with him. This illusion ends if Arinellious ends it with a bonus action, or if he takes 40 damage from a single source while in the illusory form.

Actions

Multiattack: Arinellious makes two attacks: both with his longsword.

Multiattack: Arinellious makes three attacks: one with his bite and two with his claws.

Longsword: Melee Weapon Attack: +11 to hit, reach 5 ft., one target. Hit: 10 (1d8 + 6) slashing damage, or 11 (1d10 + 6) slashing damage if used with two hands, plus 9 (2d8) psychic damage.

Claws (Illusory Form Only): Melee Weapon Attack: +11 to hit, reach 5 ft., one target. Hit: 8 (1d6 + 5) slashing damage plus 13 (4d6) psychic damage.

Bite (Illusory Form Only): Melee Weapon Attack: +11 to hit, reach 5 ft., one target. Hit: 14 (2d8 + 6) Piercing damage plus 18 (4d8) psychic damage.

Counterspell(3/day). Arinellious casts counterspell at 3rd level.

Enter the description for how legendary actions work for your monster here.

Legendary Action Name (Costs # Actions). Enter the description for your legendary action.

Arinellious 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. Arinellious regains spent legendary actions at the start of its turn.

Detect Thoughts: Arinellious casts detect thoughts.

Mind Blast (Costs 2 Actions): Arinellious unleashes a psionic blast in a 30-foot cone. Each creature in that area must succeed on a DC 23 Intelligence saving throw or take 24 (7d6) psychic damage and be stunned for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.

Dispel Magic (Costs 2 Actions): Arinellious ends one spell or magical effect on a creature or object it can see within 120 feet of it. The level of the spell or magical effect doesn't matter, but the target must succeed on a DC 23 ability check using its spellcasting ability or the effect ends.

Mind Control (Costs 3 Actions): Arinellious targets one creature it can see within 60 feet of it. The target must make a DC 23 Wisdom saving throw. On a failed save, Arinellious takes control of the target's mind and can issue commands to it verbally. The effect lasts for 24 hours, until Arinellious releases the target, or until the target takes damage. Arinellious can have up to three creatures under its control at one time.