Demon Smoke

Demons appear as smoke in their incorporeal form; they manifest by possessing humans, alive or recently deceased, via demonic possession. Acheri demons are the only exception, as they are able to manifest from their smoke form into that of a little girl. When an exorcism is performed, this forces the demon to leave its vessel or meatsuit, and the smoke is usually seen pouring out of the person's mouth. In nearly all cases the smoke is black, although Crowley's smoke was shown to be red..

Abaddon possesses the ability to partially enter a person, by sending a piece of her "smoke" form into the person's body, to read their thoughts.

Episodes

1.04 Phantom Traveler

An unnamed demon with an affinity for causing disasters, begins possessing people and causing commercial airliners to crash. This demon showed different attributes than demons that would be seen later, for instance: appearing as more of a cloud of dust than smoke, having the ability to enter a person through their eyes and violently reacting to the word "Christo."

1.22 Devil's Trap

After luring Meg into a devil's trap in Bobby's home, Sam and Dean try and get information out of her regarding the whereabouts of John. When it becomes clear that Meg is not going to be forthcoming with them, they threaten an exorcism, which gets her to finally give up all she knows about John. Satisfied that she is telling the truth, Dean orders Sam to finish the exorcism, much to Meg's displeasure. Though Bobby warns them that doing so will kill the girl being possessed, Dean justifies it as putting her out of her misery. As Sam finishes the exorcism, Meg throws her head back and screams, the demon leaving through her mouth in a black cloud of smoke and spreads out in the devil's trap in ceiling before disappearing.

After Sam shoots an Azazel possessed John in the leg. John begins pleading and ordering Sam to finish the job while he is still in control, much to Dean's protest. Before Sam can make a choice, Azazel smokes out of John's body and escapes through the floorboards of the shack.

2.01 In My Time of Dying

After the Impala is run off the road, the demon steps out of the semi-truck and rips off the door of the Impala, only to be met with Sam pointing the Colt at it. The demon tells Sam that he is bluffing, but when Sam cocks back the hammer on the Colt, it is enough for the demon to exit his meatsuit and make its escape into the night