Devils are a recurring high-level antagonistic force in Dungeons & Dragons, being the Lawful Evil opposite of the Chaotic Evil Demons - whom they are at near-constant war with and thus both utilize mortal souls as a means to try and win the war, it is believed by many sources that if either Devils or Demons won the war the end of all realms would begin as the victor would finally have the power to flood all other realms with their particular branch of evil.
Devils are a type of Fiend (just like their hated enemy, the Demons) and reside in the Nine Hells - a realm that embodies lawful evil, as such all inhabitants of said realm are also included towards lawful evil behavior and ideals.
This makes Devils natural bullies, slavers and taskmasters who seek conquest and oppression but do so in a methodical pattern - they will make deals with others (as long as the deals are in their own favor) and obey the orders of those stronger than themselves, at least until they can find a means to gain power themselves (again, within the "rules" of lawful evil).
The inclusion of Devils (alongside Demons) in early editions of Dragons & Dragons contributed to the false assumption by certain groups that the game was actually Satanic in nature - this panic caused Wizards of the Coast to lower the appearance / references to Devils and Demons for a short while but as of the modern edition they are back as one of the most prominent threats higher-level parties may encounter.
Abishais: Dragon devils who were mortals that Tiamat saw use in. They are modeled after the five chromatic dragons and take traits from certain player characters. Black are rogues, blue are wizards, green are sorcerors, red are fighters, and white are barbarians.
Amnizus: Infernal generals that take the forms of green bloated, hairless wizards with wide mouths and wings protruding from their shoulders. The are notorious for being intelligent and arrogant.
Barbazus (Bearded Devils): Shock troop fiends with beards that are tipped with barbs that inject poison into whatever they pierce. They respond with violence to anything.
Cornugons (Horned Devils): Tall golden fiends with goblin-like faces and skin as hard as metal. They are notoriously lazy, (which may be a call back to the sin of sloth) but when they are in the fighting zone, they can hurl fire balls and pierce their opponents with their tails which can bleed a man dry.
Erinyes: Attractive devils that bear humanoid looks and were originally angels who fell from grace out of misdeed and/or temptation. They sport plate armor and some weild ropes of entanglement.
Gelugon (Ice Devils): Tall insectoid fiends that mostly inhabit the icy hell layers. As powerful as they are, they are not satisfied in being second in command devils and lust for the power their superiors possess.
Hamatulas (Barbed Devils): As their simple name implies, these fiends are covered with long sharps prongs. They are exceptionally greedy and lustuous. Fighting these things is a dangerous deed whether in close or far range, as they fling fireballs and have deadly claws.
Imps: Tiny non-combative devils that are deniznes of all the Nine Hells. They act as spies and/or familiars to their masters. They can take the forms of a rat, a raven, or a spider to cover up.
Kytons (Chain Devils): These sadistic fiends only wear chains as clothing and act as jailers and torturers for the damned souls unlucky enough to end up in the hell they reside in.
Lemure: Lemures are the starting form of any one who's soul made it to any layer of hell. They are deformed, weak, and can only speak through babbling.
Merregon: Results of a fallen warriors whom archdevils find use in so they are shaped into faceless, muscular caped fiends with an expressionless faced mask bolted onto their heads. They express no fear whatsoever and therefore, do not back out of a mission.
Narzugon: Results of paladins who made deals with archdevils and carrry on their loyalty to them after death. A narzugon's flaming lance can send the soul of anyone unlucky enough to be slain by it into the hell that the narzugons hail from.
Nupperibo: These creatures are souls of those who are too worthless to even be a lemure. They take the forms of bloated gluttonous mounds of flesh with limbs and a head. Their facial orifices are sewn shut.
Orthon: Bounty hunting devils with magic crossbows that deliever many different deadly effects of the owner's choice. They have sharp sences and can turn themselves invisible. They also enjoy giving their victems a sporting chance, as it makes the experience more fun for them. They resmble tall monstrous orcs.
Pit Fiends: These are the generals of the Nine Hells that only rank below archdevils. They are the leaders of infernal legions. They take a great resmeblence to the stereotypical assumption of what the devil from theology looks like. Tall red fiends with dracon wings.
Spinagons (Spined Devils): Short devils of the infernal legions. They are simple, but effective. They shoot spines from any part of their body that grows any. The projectile explodes against any surface it contacts. They act as mennegers and intellect gatherers in the Nine Hells.