Demonology

Demonology is the study of demons or beliefs about demons.

General traits of Demons in the Supernatural Verse

Demons were originally human, but during their time in hell they lose all trace of their humanity.
Appear as a ‘black smoke’ in incorporeal form
Presence may leave traces of sulfur
Manifest by possessing humans
Can resurrect people but only if a deal is made
- A possessed human will be unaffected by physical harm, until dispossessed
Exorcism will drive the demon from its human host and send it to hell
Have a range of abilities including mind control, telepathy, telekinesis and super strength
Up until Season 3 the only method of permanent dispatch was to shoot them with the Colt. In episode 3.01 The Magnificent Seven the character Ruby introduced a Knife that could kill demons as well. In episode 3.04 Sin City Bobby rebuilds the Colt with Ruby's help.
Humans possessed by demons exhibit black, red or yellow eyes depending on the demon involved.

Eyes turn entirely black during possession, though this can seemingly be turned on and off.
They are impervious to hallowed ground ("that may work in the minor league, but not with me" - Meg, Salvation), but not to Holy Water
They can be exorcised and thus the demon returned to Hell
Possession can occur by physical contact
Restricted by the salt barrier
Vulnerable to the Colt.
Vulnerable to the blade owned by Ruby
Can resist exorcism (Meg!Demon in 2.14 Born Under a Bad Sign)
Can trap themselves within their human host using a binding spell
Can communicate

Tammi - the black-eyed demon in 3.09 Malleus Maleficarum - appears to be more powerful than other black-eyed demons. Similar to Azazel, she can pin both Sam and Dean to the wall, and use her power to kill at a distance. She causes internal bleeding in Dean, similar to that seen in 1.22 Devil's Trap. Tammi stops a bullet from The Colt when Sam tries to shoot her.

The Crossroads Demon makes bargains with people, granting a wish in exchange for claiming their life and soul.

she has red eyes
makes deals with
- Robert Johnson in 2.08 Crossroad Blues in 1930
- George Darrow, Sean Boyden, Silvia Growman and Evan Hudson in 1996 in 2.08 Crossroad Blues
- Dean in 2.08 Crossroad Blues
- Dean in 2.22 All Hell Breaks Loose: Part Two
in 3.05 Bedtime Stories she reveals that 'someone higher' than her holds the contract and describes herself as 'a saleswoman'
is killed by Sam with the Colt in 3.05 Bedtime Stories.

Presence in an area will be presaged by cattle deaths, electrical storms and temperature fluctuations.
Makes clocks stop, and electical devices go haywire
Is impervious to Holy Water
Has yellow eyes (iris and pupil)
Impervious to the salt barrier
Vulnerable to the Colt
Can also possess although supernatural beings such as the Reaper
Casey in 3.04 Sin City claimed he was Azazel

Meg Masters refers to the Yellow-Eyed Demon as "Father" ^{1.11 Scarecrow}
In 1.22 Devil's Trap, the Yellow-Eyed Demon refers to Meg as his daughter, and Tom as his son.

Acheri Demon: summoned by Ava in against the other Special Children. Sam says they are demons that manifest as children. He dispels it using an iron road. It cannot cross a salt barrier.^{2.21 All Hell Breaks Loose: Part One}