A demon (or daemon, from Ancient Greek, δαίμων), is a supernatural being from various religions, occultisms, literatures, and folklores that is described as something that is not human and, in ordinary (almost universal) usage, malevolent.
Demons are human souls that have been tortured in hell and turned into evil heartless creatures. The first demon was Lilith, created by Lucifer. The species as a whole function as the series' primary antagonists and are the most recurring creatures in the series.
The Secret Circle
The origin of the Demonic race remains a mystery, but it is known Dawn Chamberlain and her coven once summoned a powerful Demon named Abaddon, presumably to grant them stronger powers
Buffy The Vampire Slayer
A Demon, also called a Hostile Sub-Terrestrial, was the designation for a number of creatures native to various Hell dimensions separated from Earth and generally considered to be evil due to their lack of human souls and their destructive tendencies.