Demons (Supernatural)

Demons are malevolent spirits created by Lucifer. They are created from corrupted human souls after extensive torture in Hell by Alastair and other demons. They become corrupted and extremely evil, and also very powerful, in this process. The species as a whole functions as the series' primary antagonists and are the most recurring creatures in the series. Similar to Angels, they require a Vessel to walk the earth, though are able to roam in their smoke form.

They are most common antagonists of the Supernatural Show aside monsters, which includes Azazel.

Physical appearance
Demons' true forms when not possessing humans are almost never seen. In All Hell Breaks Loose - Part 2, however, after John's ghost pulls Azazel out of his vessel, a decaying skull-like face is visible within Azazel's smoke form. This is arguably also seen in demons stabbed with the demon knife or an angel blade, when their vessels flash orange and the skeleton beneath is visible. From this statement, it can be assumed that demons' true form, in fact, a skeleton that engulfed by smoke.