Short answers, pulled from the story.
A devil, also called a baatezu, is a Lawful Evil creature native to the Nine Hells of Baator in the Dungeons & Dragons roleplaying game. Devils rank through a strict hierarchy from lemures up to the Archdevils, the Lords of the Nine, who each rule one of Hell's nine layers.
TSR renamed devils to baatezu, and demons to tanar'ri, in 2nd edition after a televangelist called the Deities and Demigods sourcebook exactly like witchcraft. TSR feared the comparison would fuel religious protests against the game. The original terms returned in 3rd edition in 2000, though baatezu remains the name of one specific subset of devils.
Asmodeus is the King of the Nine Hells of Baator and the Overlord of the lesser Dukes of Hell in Dungeons & Dragons. He appears as a giant human over 13 feet tall with dark skin, red eyes, and small horns, and he carries a unique artifact called the Ruby Rod.
The Blood War is the centuries-long conflict between devils and demons across the Dungeons & Dragons multiverse. Devils treat the mortal worlds of the game as tools to be exploited, partly in service of this war.
Zariel is the current ruler of Avernus, the first layer of the Nine Hells of Baator, in Dungeons & Dragons. She is a fallen angel of Celestia who was deposed by the pit fiend Bel, later reclaimed her throne after Bel mismanaged the Blood War, and was placed in charge of Avernus by Asmodeus.
Devils first appeared in the original 1977 first-edition Advanced Dungeons & Dragons Monster Manual. That book introduced creatures including the barbed devil, the bone devil, the erinyes, the pit fiend, and the archdevils Asmodeus, Baalzebul, Dispater, and Geryon.