Bifrons

Aura of Fearful Obedience.When a Devil within 20 feet of Bifrons takes the Multiattack action, it can make one additional attack.

Master Wizard. Bifrons ignores all rules that limit the amount of levelled spells he can cast in a turn.

Bifrons' Bejewelled Robes. These robes, set with rubies, grant Bifrons an armor class of 25 and a flying speed of 60 feet, with which he can hover.

Staff of Infernal Glyphs. Bifrons' staff grants him a +2 bonus to spell attack rolls and spell save DCs. Without it, they are +15 and 25, respectively.

Spellcasting. Bifrons is a 22nd-level spellcaster. Its spellcasting ability is Intelligence (spell save DC 27, +17 to hit with spell attacks). Bifrons has the following spells prepared (in no particular order):
At will: Sleet Storm, Otiluke's Freezing Sphere, Wall of Ice, Dispel Magic (as a 9th-level spell), Wall of Force, Globe of Invulnerability

3/day: Antimagic Field, Gate, Armour of Agathys (as an 8th-level spell), Disintegrate, Dimension Door

1/day: Canian Winds (10th-level spell), Mephistopheles' Storm (9th-level spell).

Innate Spellcasting. The pit fiend's spellcasting ability is Charisma (spell save DC 21). The pit fiend can innately cast the following spells, requiring no material components:

At will: Detect Magic, Fireball, Heroism (as a 9th-level spell)

3/day each: Hold Monster, Wall of Fire

Legendary Resistance (4/day). When Bifrons fails a save, he can choose to succeed instead.

Drain Power. At the start of Bifrons' turn, if there is an Infernal Glyph on the battlefield, he may end its effect and regain all spent uses of Legendary Resistance.

Quickened Spellcasting. Bifrons casts up to two spells, but both must be At-Will spells (these spells can be the same or different ones). Alternatively, he casts an At-Will spell and then makes two Polar Ray attacks.

Infernal Glyphs (Recharge 4-6). Bifrons slams his Staff into the ground, creating two of the following glyphs on the ground within range. Each glyph has AC20 and 40 HP; breaking it ends the effect. Bifrons may also use these glyphs to recharge Legendary Resistances; see Drain Power.

• Glyph of Icy Winds. All enemies of Bifrons on the battlefield are in Difficult Terrain until the Glyph is destroyed. Additionally, all creatures are Deafened and immune to Thunder damage.
• Glyph of Cold Resolve. Whenever an ally of Bifrons attacks, that attack gains advantage.
• Glyph of Selective Antimagic. The area within 40 feet of the glyph is under the effects of an Antimagic Field spell. Bifrons can ignore this effect when casting spells.
• Glyph of Order's Command. Devils on the battlefield are considered to be within range of Bifrons' Aura of Fearful Obedience ability.
• Glyph of Wintry Doom. At the start of Bifrons' turn, if this glyph is on the battlefield, all creatures take 100 (20d10) Cold damage.

Multiattack. Bifrons makes four Polar Ray attacks.

Staff of Infernal Glyphs. Melee Weapon Attack: +12 to hit, reach 10 ft., 1 target. Hit: 65 (5d10 + 10) Force damage. 

Polar Ray. Ranged Spell Attack: +17 to hit, range 150 ft., 1 target. Hit: 34 (6d8 + 9) Cold damage.

Quickened Dimension Door (Recharge 5-6). Bifrons casts Dimension Door.

Bifrons may take up to 2 reactions per round, but no more than one per turn.

Turn Magic. If a creature casts a spell that requires a saving throw on Bifrons, and he passes the save, he may immediately cast that spell on a target he can see within range, using the original caster's spell save DC and attack modifiers.

War Caster. When a creature leaves his reach, Bifrons may cast a spell that targets only that creature as a reaction.

Bifrons may take up to 4 legendary actions, but no more than one per turn.

Let them be an example! Bifrons may take this legendary action at the end of a turn in which another Devil died. Each Ice Devil on the battlefield may immediately use its reaction to move up to its speed and make two attacks.

Your petty rebellion has failed. Bifrons may take this legendary action at the end of a turn in which a creature attacked him. He can roll up to five of his Hit Dice, regaining that many HP, and one of the creatures that attacked him must make a DC20 Wisdom saving throw or immediately move 20 feet away from Bifrons.

My arcane power far outstrips yours! Bifrons may take this legendary action at the end of a turn in which an enemy cast a spell. He immediately casts any spell which he could cast at will, excluding Dispel Magic.

None dare ignore me. Bifrons may take this legendary action at the end of an enemy's turn in which they did not interact with him whatsoever. He immediately either makes two Polar Ray attacks or recharges Infernal Glyphs.