Pit fiend

Pit fiends are mighty devils ranking just below the archdevils themselves.

Society
Pit fiends are the generals of the Nine Hells, and are high-ranking within the hierachy of hell. They are considered the servants of the archdukes and archduchesses of the realm, but of a higher station than most other devils. They are manipulative and controlling, and their main goal is to eliminate any obstacles in their way.

Abilities

 * Immunities:
 * Damage Resistance: Cold; Bludgeoning, Piercing, and Slashing from Nonmagical Attacks that aren't silvered.
 * Damage Immunities: Fire and Poison.
 * Condition Immunities: Poisoned.
 * Fear Aura: Any hostile creature can be frightened.
 * Magic Resistance: The pit fiend has advantage on saving throws against spells and other magical effects.
 * Magic Weapons: The pit fiend's weapon attacks are magical.
 * Innate Spellcasting: Can cast a few spells which require no material components.
 * Attacks:
 * Multiattack: The pit fiend can make four attacks: one with its bite, one with its claw, one with its mace, and one with its tail.
 * Bite: The target can become poisoned. While poisoned in this way, the target can't regain hit points.
 * Claw: The fiend can make a claw attack.
 * Mace: It can make an attack with its Mace.
 * Tail: The pit fiend can attack with its tail.

Campaign One Arc 3: The Chroma Conclave
Vox Machina traveled to the Elemental Plane of Fire in hopes of retrieving the Plate of the Dawnmartyr to aid in their upcoming battle against Thordak. The owner of the Plate, Juuraiel, tasked them with killing the Nine Hells' ambassador in the City of Brass, a pit fiend named Ghurrix, who had killed Juuraiel's lover. They succeeded in killing Ghurrix and his summoned erinyes, and narrowly avoided being arrested. Ghurrix was assuredly demoted for being killed by what appeared to be a random group of mortals.

Campaign One Arc 4: Taryon Darrington
In their quest to infiltrate the Mentiri to destroy Hotis, Vox Machina accepted a contract from Ipkesh to assassinate a gluttonous pit fiend named Utugash. They followed through with the help of Tova, a werebear blood hunter who had been purchased and tortured by Utugash.

Explorer's Guide to Wildemount
The epic-level adventure hook Dark Days features a pit fiend named Vanatoth leading an invasion from the Nine Hells into the Barbed Fields.

Notable pit fiends

 * Utugash: Assassinated by Vox Machina in the City of Dis, where he had Tova captive.
 * Ghurrix: An ambassador of the Nine Hells who frequented the slave markets in the City of Brass. Also assassinated by Vox Machina.
 * Izeelzee: The pit fiend whose life force Bane used to forge The Bloody End, one of the Arms of the Betrayers.