
The possessed humans also gain the powers of whichever demon is possessing them.

Upon exorcism the human - if they survived - retains no memory of their time during possession (unless they have been "awake" during possession, in which case the time of possession is a nightmare for them). Often under possession the human begins speaking and acting like the demon would. Angels and other supernatural beings can see the true faces of demons underneath their vessels. Whenever they use their powers, they leave a trace of sulfur in the form of yellow dust. Demon possession also causes a chemical change in the circulatory system of the host demon blood is known to be a unique serum and has certain attributes when ingested. Possessed humans often look exactly the same as they did before possession, the only noticeable change being that their eyes will sometimes change color (black, red, white, grey, or yellow depending on the demon possessing them). On one occasion, a demon entered a human via the eyes (as seen in the episode Phantom Traveler). When they possess humans, they usually enter through their mouths. While nearly all demons desire to escape Hell any chance they get, some demons actually prefer Hell, like Alastair, and even have never left Hell after becoming a demon like Belphegor. To do this, they have to take a vessel or "meatsuit" in order to physically exist.

Some demons are sent back to Earth to accomplish certain tasks, such as causing disasters, spreading diseases, assassinating hunters, or manipulating people into committing crimes that will get them sent to Hell. Very powerful demons leave much more sulfur than average, as evidenced when Azazel kidnapped Sam. This can give away the demon's presence, indicating to hunters that they are hunting a demon. Often when a demon has used its powers, they leave a trace of sulfur in the nearby space, or if on a person, around the victim. Once a soul becomes a demon, it gains the basic demonic powers ( possession, superhuman strength, etc.) and from there, it can work its way up the ladder to positions of greater authority and power within Hell. He says they look horrible even going as far to call Ruby "one ugly broad." Their true forms have not been shown onscreen, however Dean has seen their true forms, before he was taken to Hell. Crowley appears as the only exception, appearing as a blood-red cloud. They manifest on Earth in the form of a long, slender cloud of black smoke. There are many ways of becoming a demon, from making a deal with a Crossroad Demon, to being a witch, and making a deal with another demon. Some demons, however, are true believers in Lucifer's agenda to "cleanse" Earth, and actually consider themselves morally superior to humans. In general, demons engage in torture and destruction because it is fun or to avoid being tortured themselves. Some have even forgotten that they were once human. According to Ruby, most demons forget what it means to be human.

Demons are the result of human souls going to Hell and being tortured until no humanity is left in them, thus turning into demons - dark, malicious spirits that revel in pain, chaos, and death.
