Demons

Demons are a species of dark-supernatural beings. Demons are fallen angels who fell from heaven and became one with the darkness, thus transformed into a new species. A demon-like species can be created from humans sent to Hell.

History
Their background is mostly shrouded in mystery, however the third season episode Malleus Maleficarum reveals that some demons were once human, their souls having been corrupted after spending hundreds of years in Hell, leaving them as large, black clouds that generate electromagnetic interference. Subsequent seasons expands their history further by tying it into Christian mythology, explaining that the first natural demon was Lilith, whom Lucifer himself corrupted as revenge against God. However, Lucifer was soon locked away by the archangel Michael, and Lilith was imprisoned deep within Hell. Thousands of years later, after much searching, the demon Azazel was able to find the doorway to Lucifer's prison. The demon revealed that only Lilith can break the 66 seals keeping him imprisoned, and instructed Azazel to find him "a very special child". As depicted in the second season finale All Hell Breaks Loose, Azazel spent the following years feeding his own blood to babies—one being main series protagonist Sam Winchester—as they would then grow up developing demonic abilities.

Although an ordinary demon is featured as a villain of the week in the fourth episode of the series, Phantom Traveler, demons do not become actively involved in the show until the introduction of Azazel's daughter—possessing a girl named Meg Masters—in the second half of the season. She follows Sam and Dean Winchester, and later unsuccessfully sets