Demons in the Supernatural universe are the end product of human damnation. Every demon began as a human soul that, upon arriving in Hell, was subjected to centuries or millennia of torture until its humanity was stripped away and something new — something cruel, powerful, and driven by malice — emerged.
The Hierarchy of Hell
Hell operates with a loose but recognizable hierarchy. At the bottom are the black-eyed demons, the rank and file who serve as soldiers, bureaucrats, and enforcers. Crossroads demons (some with red eyes) operate the deal-making apparatus, collecting souls through bargains. White-eyed demons (Lilith, Alastair) are among the oldest and most powerful, predating most other demons by millennia. Knights of Hell (Cain, Abaddon) are an elite class created directly by Lucifer. The King of Hell sits at the top — a title held most notably by Crowley, a former crossroads demon who clawed his way up through ambition and cunning.
Possession & Detection
Demons require human hosts to operate on Earth. Possession is non-consensual — the demon simply enters the body and takes control. The host is conscious and aware but unable to act. Detection methods include: holy water (causes burning), reaction to the name "Christo" (causes flinching in some lore), devil's traps (immobilizes), and anti-possession tattoos (prevents entry).
Key Figures
Azazel (the Yellow-Eyed Demon) orchestrated the Winchester family tragedy. Lilith was the first demon ever created, made by Lucifer from a human soul. Crowley became the longest-running demon antagonist, eventually dying heroically. Ruby manipulated Sam Winchester as part of a long-term plan to free Lucifer. Meg, originally an antagonist, eventually became a reluctant ally.