Devil

Devils are lawful evil fiends from the Nine Hells and are ruled by Asmodeus, the Lord of the Hells.

Society
Devils, in contrast to demons, are creatures of order and tyranny, and they are organized in an extremely rigid hierarchical manner. Their power structure is based on gaining sway over mortals' souls, and their bread-and-butter for gaining influence and power is through contracts. Many devils' whole purpose is to figure out how to gain power and rise in rank by proving themselves or tearing those above them down. Given that devils are constantly trying to undercut one another, they thrive on paranoia and deception. In the meantime, they serve more powerful devils to gain favor, as higher devils can give lower-ranked devils the chance to rise to a higher tier. Devils can lose rank by showing weakness. Whereas few mortals visit the savage demon-ruled Abyss, mortals can conduct business in the Nine Hells, mostly in the Iron City of Dis.

Percy's summary is that devils are vicious, psychopathic creatures that only function because they maintain these rules with each other about how they're supposed to act.

Somewhat outside the system are a few other kinds of fiends: succubi, incubi, and notably rakshasas, though rakshasas are devils who originated in the Nine Hells.

Devils, among other creatures that can subsist on and gain power from souls, sometimes consume soulstones.

Types of devil
This list is arranged by hierarchy.

Archdevils
The rulers of the Nine Hells are the Archdevils, chief among whom is Asmodeus. Among the Archdevils beneath him is Dispater, who rules over Dis, the second layer of the Nine Hells, which includes the Iron City of Dis.

Archdevils approach close enough to divinity that they cannot cross the Divine Gate on their own, though sufficient mortal magic can summon them to the Material Plane.

Greater devils

 * Pit fiend
 * Erinyes
 * Horned devil

Lesser devils

 * Osyluth (Bone devil)
 * Chain devil
 * Hamatula (Barbed devil)
 * Bearded devil
 * Nergaliid
 * Spined devil
 * Imp

Lemures
Lemures have the lowest rank in the Hells; they are ever-tortured souls who have fallen under the sway of a devil, and are used as energy or as slaves. If they manage to please their masters, perhaps through sufficient service over an extended time, they may ascend to the ranks of the lesser devils.

History
At least on Exandria, most research on devils is to be found in the grasp of the Cobalt Soul, according to Highbearer Vord.

The Founding
During the Founding, some of the celestials sent to fight the demons of the Abyss fell into hate and tyranny; one fallen angel, Asmodeus, corrupted other celestials into the first devils, and under his leadership they forged the Hells. An alternative origin for devils is that the Betrayer Gods imposed order on demons to create armies of devils. A conflict called the Blood War has raged between these devils of the Hells and the demons of the Abyss ever since.

The Age of Arcanum
Before the Calamity, eight of the Betrayer Gods each forged a sentient weapon; three of these, made by Asmodeus, Tiamat, and Bane, each contain the life force of a devil.

The flying city of Aeor created Aeorian hunters to take on, among others, devils.

The archmage Halas made many deals with many devils, but wormed his way out of every contract; he bound a horned devil in the Circlebound Chamber of the Folding Halls of Halas. The Chamber has tapestries depicting the Blood War.

The Calamity
During the Calamity, at least one archdevil was active on Exandria: Asmodeus, with his phoenix companion and mount Desirat. Asmodeus betrayed Sarenrae and wiped out most or all of her followers in one stroke, but he was eventually defeated by Avandra (who tricked his armies into fighting each other), and Desirat was torn from her master and bound beneath Mount Mentiri by the remnants of the original Cerberus Assembly.

Scattered War
Neminar Drassig dabbled in infernal magic and made a pact with Bane that apparently continued through his younger brother Trist. Though the main description of the Battle of the Umbra Hills mentions only demons aiding Trist's army, devils apparently also participated.

Marrow War
When the Dwendalian Empire invaded the Julous Dominion in 544 PD, the high cleric of the Julous Dominion made a pact with a devil of the Nine Hells to protect against the Empire. Devils and undead swarmed the Marrow Valley, which did offer protection against the Empire but also menaced the residents. The Raven Queen gave one of her priests, Trence Orman, knowledge of blood magic, which he spread to trusted friends. So began the Claret Orders. The Claret Orders purged the valley of the fiends and undead, and Dwendalian forces promptly swept in.

Pre-Stream

 * When Vox Machina investigated the home of Sir Gregory Fince, they found him slain in the basement, and encountered an imp in the upstairs library.
 * A glabrezu (treachery demon) initially passed himself off as a "devil prince" by the name of Juurezel when confronted by Vox Machina. He informed them of the great war between demons and devils.

Arc 1: Kraghammer and Vasselheim
Half of Vox Machina was contracted, under the auspices of the Slayer's Take, to work with Kashaw Vesh and Thorbir Falbek to find and destroy a rakshasa, which they quickly did.

Arc 3: The Chroma Conclave
After Thordak escaped from the Elemental Plane of Fire and razed Pyrah, imps were among the creatures wandering through the widened, unguarded rift into the Sunderpeak Mountains. Vox Machina didn't encounter any by the time they returned to the Cindergrove.

The previously slain rakshasa, Hotis, returned from the Nine Hells and attempted to kill Vax'ildan, but was once again slain himself.

Vox Machina noticed a heavy devil presence openly conducting business in the City of Brass. To earn their way out of slavery, the party assassinated the pit fiend Ghurrix who served as ambassador from the Nine Hells, and they also dispatched the erinyes he summoned.

Arc 4: Taryon Darrington
To prevent Hotis from returning to attack them again, Vox Machina conducted research on rakshasas and devils and traveled to the Iron City of Dis to destroy Hotis once and for all. They naturally encountered many devils, as well as the half-devil cambion Ipkesh.

Arc 5: Vecna
Demistrik was an imp living with Sprigg after the imp's master died trying to get into Sprigg's home. When that house was destroyed, he quickly made his way to another home in Kraghammer.

Arc 1: Come Together
The Mighty Nein discovered that one of the members of the Fletching and Moondrop Traveling Carnival of Curiosities was in fact a nergaliid, a devil toad, who summoned imps to aid him.

Arc 4: Swords and Angels

 * Two imps reside in Ava Endlewood's shop in Uthodurn. Reani knows about them, but Samliel hasn't told her to kill them.
 * The Mighty Nein talked with the horned devil bound by Halas; it had apparently gone mad from extended isolation and had forgotten its name, and pleaded with them to read a "poem" in a green book to help it remember things.
 * The Nein also fought against Obann, who had been a devil but who changed into a demon by allying himself with an entity of the Abyss (Tharizdun).

Arc 5: Family Ties
Fjord inadvertently discovered an imp in Vess DeRogna's tower.

Notable devils

 * Asmodeus, Lord of the Hells
 * Dispater, archdevil of the second layer of the Nine Hells
 * Ghurrix, a pit fiend serving as ambassador to the Elemental Plane of Fire until assassinated by Vox Machina on behalf of Juuraiel
 * Utugash, a pit fiend who bought Tova to torture her and was ultimately assassinated by Vox Machina on behalf of his subordinate Ipkesh
 * Hotis, a rakshasa who became a repeat threat to Vox Machina until they went to the Nine Hells to destroy him for good
 * Obann, once a devil, who became a demon by long service to a creature of the Abyss
 * Kylre, a nergaliid who traveled with the same circus as Mollymauk Tealeaf and Yasha Nydoorin, slain by the Mighty Nein in their first adventure together