Valgavoth, Terror Eater (/u/letterephesus)

Fear Aura. Any creature hostile to the pit fiend that starts its turn within 20 feet of the pit fiend must make a DC 21 Wisdom saving throw, unless the pit fiend is incapacitated. On a failed save, the creature is frightened until the start of its next turn. If a creature's saving throw is successful, the creature is immune to the pit fiend's Fear Aura for the next 24 hours.

Limited Magic Immunity. Valgavoth can't be affected or detected by spells of level 6 or lower unless he wishes to be. He has Advantage on saving throws against all other spells and magical effects.

Legendary Resistance (5/Day). If Valgavoth fails a saving
throw, he can choose to succeed instead.

Magic Weapons. The Demon's weapon attacks are magical.

Discorporation. If Valgavoth is killed, his essence returns to
his lair and is transferred to a cocoon that has Immunity to all
damage. After 1d10 years, Valgavoth regains all his Hit
Points and emerges from the cocoon.

Innate Spellcasting.  Spellcasting. Valgavoth casts one of the following spells,
requiring no Material components and using Charisma as the
spellcasting ability (spell save DC 27):
At will: Detect Magic, Dispel Magic, Fear
3/Day Each: Geas (level 7 version), Simulacrum (as an
action)
1/Day Each: Reverse Gravity, Time Stop

Multiattack. The pit fiend makes 2  rend attacks and uses consume fearl.

Rend. Melee Weapon Attack: +19, reach 10 ft. Hit: 23
(2d12 + 10) Force damage plus 11 (2d10) Psychic damage.

Consume Fear. Wisdom Saving Throw: DC 27, one creature within 120 feet. Failure: 23 (2d12 + 10) Psychic damage, and the target has the Frightened condition until the end of its next turn. Valgavoth gains Temporary Hit Points equal to the Psychic damage taken. Success: Half damage only.

Web Breath (Recharge 5–6).. Dexterity Saving Throw: DC 26,
each creature in a 300-foot Cone. Failure: 71 (11d12) Acid
damage, and the target has the Restrained condition until the
web is destroyed (AC 17; HP 20; Vulnerability to Fire
damage; Immunity to Poison and Psychic damage). Success:
Half damage only.

Valgavoth can take up to four Reactions per round but only
one per turn.
Alter House. Trigger: A creature within 300 feet of Valgavoth
takes the Dash, Disengage, or Hide action. Response:
Valgavoth alters the terrain of the house. He can create or
vanish doors, walls, pits, and furniture.

Alter Time. Trigger: A creature Valgavoth can see ends its
turn. Response: Each creature Valgavoth can see rerolls
Initiative.

Counterspell (Recharge 5–6). Trigger: A creature within 60
feet casts a spell. Response: Valgavoth casts Counterspell
(spell save DC 23).
Rend Retaliation. Trigger: Valgavoth takes damage.
Response: Valgavoth makes one Rend attack.

Using a deck of Archenemy Scheme cards from Duskmourn, House of horror commander, homebrew your own set of villain actions. 
Every action-oriented creature has three villain actions they can use after an enemy’s turn. Villain actions are simi lar to legendary actions with the following exceptions: • A creature can use only one villain action per round (as such, villain actions tend to be more powerful than legendary actions). • Each villain action can only be used once during a combat encounter. Like legendary actions, a creature can’t use villain actions if incapacitated or otherwise unable to take actions

Each trio of villain actions has a recommended round order. These abilities give the battle a logical flow and a cinematic arc: • The first villain action is an opener, which shows the characters they’re not battling a typical creature. Openers generally deal some damage, summon a lackey or three, buff the boss, debuff the characters, or move the creature into an advantageous position. They’re just a taste of what’s to come. • The second villain action provides crowd control. It typically fires after the heroes have had a chance to respond once or twice, get into position, and surround the villain. This second action helps the villain regain the upper hand. Like an opener, this action comes in many flavors, but it’s even more powerful than an opener. • The third and final villain action is an ultimate move or “ult”—a showstopper the villain can use to deal a devastating blow to the characters before the end of the battle. While every creature has a recommended order of actions, you can take villain actions in any order if it makes your f ight more dramatic. You could push back a villain action if a creature is stunned or might stay alive for more than three rounds, or you could perform the recommended third action in round two after several surprise critical hits