Demons

Demons are a species of supernatural 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, with the second being Ghosts

Characteristics
Demons are creatures, who come from Hell. Their true form is that of a black steam... When they possess humans, they enter through their mouths. Whenever they use their powers, they leave a trace of sulfur in the form of yellow dust. People suffering from stress, fear or panic can be vulnerable to demonic possession. Demons abhor the name of God (which in Latin is Christo).

Appearances
They have appeared in 52 episodes to date. Even though, one appeared in "Pilot" as a silhouette, they were formally known in "Phantom Traveler". They appeared - briefly or as "monsters of the week" - in the following chapters include "Scarecrow", "Shadow", "Salvation", "Devil's Trap", "In My Time of Dying", "Croatoan", "Born Under A Bad Sign", "All Hell Breaks Loose - Part 1", "All Hell Breaks Loose - Part 2", "The Magnificent Seven", "The Kids Are Alright", "Sin City", "Malleus Maleficarum", "Dream A Little Dream Of Me" (in one of Dean's dreams), "Jus In Bello", "Time Is On My Side" (in a flashback), "No Rest For The Wicked", "Lazarus Rising", "Are You There, God? It's Me, Dean Winchester", "In The Beginning", "Metamorphosis", "Yellow Fever" (as Dean's hallucination), "I Know What You Did Last Summer", "Heaven and Hell", "Criss Angel Is A Douchebag", "Sex and Violence" (Ruby's Voice), "Death Takes A Holiday", "On The Head Of A Pin", "The Rapture", "When the Levee Breaks", "Lucifer Rising", "Sympathy for the Devil", "Good God, Y'all" (as hallucinations created by War), "Free To Be You and Me", "I Believe The Children Are Our Future", "Abandon All Hope...", "Swap Meat", "My Bloody Valentine", "99 Problems", "The Devil You Know", "Two Minutes to Midnight", "Swan Song", "Weekend At Bobby's", "Family Matters", "All Dogs Go To Heaven", "Caged Heat", "Mommy Dearest", "The Man Who Would Be King", "Let It Bleed" and "The Man Who Knew Too Much".

Story
Demons were once human, their souls having been corrupted and warped after spending hundreds of years in Hell being tortured and having all of their humanity burnt out of them. They are left as large, black clouds that generate electromagnetic interference and leave behind a sulfurous residue. The first demon was Lilith. She was created by Lucifer, who resented humans for taking his place as the most beloved of God. Lucifer wanted to show how bad humans were, so tortured a human into becoming a demonic monster. He proceeded to do the same to any human souls he could trick into joining him. However, Lucifer was locked away by his own brother, the Archangel Michael, and Lilith was imprisoned deep within Hell. Thousands of years later, after much searching, Azazel, Lucifer's demonic "son", was able to find the doorway to Lucifer's prison. Lucifer revealed that only Lilith can break the 66 seals keeping him imprisoned, and instructed Azazel to find him "a very special child". Azazel spent the following years feeding his own blood to babies—one being Sam Winchester—as they would then grow up developing demonic abilities, including the ability to kill Lilith and hence free him from his prison, and then be powerful enough to serve as his earthly vessel.

Although an ordinary demon is featured as a villain of the week in the fourth episode of the series, Phantom Traveler, demons do not become actively involved in the show until the introduction of Azazel's "daughter"—possessing a girl named Meg Masters—in the second half of the season. She follows Sam and Dean Winchester, and later unsuccessfully sets a trap for them using their father.

Powers and Abilities
Lower level demons (Black eyed demon) powers Crossroads Demons (Red eyed demon) powers Upper-hierarchy (Yellow eyed demons and White eyed demons) powers
 * Demonic possession
 * Superhuman strength
 * Enhanced endurance and speed
 * Telekinesis
 * Magical and occult knowledge
 * Control over Hellhounds
 * Demonic possession
 * Teleportation
 * Wish granting
 * Superhuman strength
 * Control over Hellhounds
 * Demonic possession (known: Azazel, Alister, Lilith, Crowley, and Samhain)
 * Pyrokinesis (known: Azazel, Crowley)
 * Injury to objects with telekinesis (known: Crowley, Azazel,Lilith
 * Enter a human's subconscious (known: Azazel)
 * Overpowering or possessing reapers ( Known: overpowering: Alister and Azazel, Possessing: Azazel)
 * Wish making( known: Lilith,Azazel,and Crowley)
 * Injury to humans by thought ( Shown: Azazel)
 * White light demon blast (Shown: Lilith and Samhein)
 * Teleportation (Known: Azazel,Crowley,Alister, and Lilith)
 * All upper level demons have the same powers (superhuman strength, enhanced endurance and speed, occult knowledge, and telekinesis) as lower level demons up top but, are at a greater level then lower level demons.
 * Control over Hellhounds (Known: Crowley)

Weakness
Salt - Demons are unable to cross a line of salt. Salt can be used to torture demons. It can also be put into shotgun rounds to hurt and knock back demons.

Holy Water - Holy Water burns demons that come into contact with it like acid. Demons like Azazel proved to be immune to Holy Water

Iron - Demons can't touch or cross iron.

Ruby's Knife - The knife originally belonged to Ruby. It will kill a demon that is within a host if the host stabbed in a fatal place. Upper level demons are immune to the killing effects of the knife. One such demon is Alastair.

The Colt - Much like Ruby's Knife it will kill any demon that is shot while within a host if shot in a fatal place.

Bones - If the bones belonging to the demon is burned the demon will be killed much like a vengeful spirit. This can also be used to torture demons. This until recently was believed to be merely a rumor.

Exorcism - Will pull the demon out of its host and send them to hell. Hosts may not survive.

Devil's Trap - A Devil's Trap will hold a demon in place and prevent their escape. The Trap will also prevent a lower level demon from using their powers.

Angel's Touch - An angel can kill a demon by touching them on the forehead with their hand. This power will not work if the demon is too powerful or if the angel is too weak.

Special Children - Sam Winchester developed special abilities almost especially tailored to hurt or kill demons. Due to training from Ruby and ingestion of demon blood he first developed the power to pull demons out of humans. Later the powers advanced to killing demons like the knife or colt. Although unknown it is assumed the other special children may have been able to do this if they survived, trained, and ingested demon blood.

Palo Santo - It is basically the wood version of holy water/salt, when a demon is stabbed the demon screams in pain and the wound smokes. It immobilizes demons long enough for exorcism.

Hallowed Ground - Some demons (lower-level ones) are unable to enter hallowed ground.