Arioch, the Lex Talionis

Chosen of Levistus. Arioch's attacks and spells ignore resistance to cold and psychic damage, and it can attune to up to five magic items.

Evasion. When Arioch is subjected to an effect that allows it to make a Dexterity saving throw to take only half damage, it instead takes no damage if it succeeded on the saving throw, and only half damage if it fails.

Icefooted. Arioch does not suffer limited movement from any sort of ice or snow based terrain, and any liquid freezes beneath its feet.

Keen Eye. When making a ranged weapon attack with advantage, Arioch can reroll one of the dice. Additionally, Arioch critically hits on a roll of 19 or 20.

Legendary Resistance (3/Day). When it fails a saving throw, Arioch can choose to succeed instead.

Magic Items. Arioch is attuned to the following items: Amulet of Proof Against Detection and Location, Enmity, Hand of Levistus, Ring of Mind Shielding, Ring of Radiant Resistance

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

Spellcasting. Arioch is a 17th level spellcaster. Its spellcasting ability is Charisma (spell save DC 20, +12 to hit with spell attacks). Arioch has following spells prepared and can cast them without material components:
At Will: absorb elements, alter self, cone of cold, counterspell, ice storm, shield
3/day each : greater invisibility, modify memory, summon fiend (6th level), wall of ice

1/day each: imprisonment, meteor swarm (cold damage), time stop

Multiattack. Arioch makes any number of Enmity (Pistol) or Claw of Levistus attacks, up to a maximum count of 3. 

Claw of Levistus. Melee Weapon Attack: +13 to hit, reach 5 ft., 1 target. Hit: 12 (1d8 + 6) slashing damage plus 12 (3d8) cold damage and 12 (3d8) psychic damage. Upon a hit, the target must make a DC 20 Wisdom saving throw or become vulnerable to cold and psychic damage until the end of Arioch's next turn.

Enmity (Pistol). Ranged Weapon Attack: +16 to hit, range 150/600 ft., 1 target. Hit: 14 (1d10 + 9) piercing damage plus 9 (3d6) cold and 9 (3d6) psychic damage. Upon a hit, the target must make a DC 20 Strength saving throw or be pushed back 10 feet or knocked Prone (Arioch's choice).

Enmity (Rifle). Ranged Weapon Attack: +16 to hit, range 1000/5000 ft., 1 target. Hit: 45 (6d12 + 9) piercing damage plus 27 (9d6) cold damage. Upon a hit, the target must make a DC 20 Constitution saving throw or become Petrified in ice.

Enmity (Blunderbuss). All creatures within a 150 ft. cone of Arioch must make a DC 20 Dexterity saving throw, suffering 15 (3d10) piercing damage plus 27 (9d6) cold damage on a failed save, or half as much on a successful one. All creatures who fail their saving throw are also knocked Prone.

Cunning Action. Arioch can use its bonus action to perform the Dash, Disengage, Hide, or Use an Object actions.

Insightful Gaze. Arioch gains advantage on all attacks rolls against a target it can see until the end of its turn.

Stygian Step. Arioch teleports to an unoccupied location it can see within 30 feet of it. All creatures that Arioch chooses within a 10 foot radius of that location must make a DC 20 Wisdom saving throw or become Frightened until the end of their next turn.

Take Aim. Arioch gains advantage on all ranged weapon attacks until the end of its turn.

Uncanny Dodge. When an attacker that Arioch can see hits it with an attack, it can use its reaction to halve the attack's damage.

Arioch has 3 Legendary Actions, and regains all expended actions at the start of his turn.

Attack (Costs 1 Action). Arioch makes a single Claw or Levistus or Enmity (Pistol) attack.

Cast a Spell (Costs 2 Actions). Arioch casts one of its spells.

Dash (Costs 1 Action). Arioch performs the Dash action without invoking attacks of opportunity.

Summon Ice Devil (Costs 3 Actions). Arioch summons 1 Ice Devil at an unoccupied location it can see within 60 feet of it. The Ice Devil acts on its own initiative and obeys any commands Arioch gives it. Once Arioch has used this action, it cannot use it again for 24 hours.