Eve

Eve was a mysterious and powerful malevolent entity who was neither demon nor angel. She was cast into the deepest bowels of Purgatory, until she was released by Dragons, arrived on Earth, using a young virgin girl as a vessel. She was part of existence before Angels were, which gave her an advantage over them, being able to inhibit them from using their supernatural powers. Eve created the Alphas, which were the first monsters of their allotted kind. She, just as of recently, created a worm-like creature which Dean dubbed the "Khan Worm". She also just did what she called "Beta testing" and created creatures that Dean named "Jefferson Starships", which were monster hybrids that she mass produced. Her only known weakness was that of Phoenix ash. She was killed by Dean who used its ashes on her.

Physical Appearance
What Eve's true form fully looks like isn't known, however she has a gray, withered face. Apparently not even Bobby knows her true form.

Early Life
Eve is older than Angels and was last on Earth 10,000 years ago. She is also known as the Mother of All, which refers to her creation of all supernatural beings. She was in Purgatory, where the souls of the supernatural go after death, until Dragons raise her

Season 6
In Family Matters, she is alluded to by the Alpha Vampire, who is being interrogated by Samuel Campbell. When Dean and Sam have a few moments alone with the Alpha Vampire, Sam asks him who made him, and he mentions that everything has a mother, even creature like him.

In Like A Virgin, Dragons are abducting young women (virgins) and keeping them captive in a sewer. Dean obtains the Sword Of Bruncvik and he and Sam track down the dragon's lair, finding a pile of gold, and the women. They also find an ancient manuscript which speaks of freeing Mother from Purgatory. Sam and Dean kill one of the Dragon but one escapes. He meets up with another dragon who has also captured a half dozen women. They take one of the women and use an incantation to open a door to Purgatory. They throw the woman in, and she rises again, her body now inhabited by Eve.

In ...And Then There Were None, Eve creates a being - called colloquially the Khan Worm - that can enter a person's body through the ear and control their actions. She plants it inside a trucker, who goes on to kill his family. It then passes to one of his co-workers who kills 12 people. While investigating the murders, it infects Dean who kills his cousin Gwen, and then infects Samuel who was killed by Sam, and finally Bobby who kills Rufus while possessed.

While possessing Bobby, it tells Dean and Sam that it was created by Eve, who intends for supernatural beings to take over the world. She also has a message for them that the worm relates: "She's pissed, she's here, and it's going to be nothing but pain for you from here on in."

In Mommy Dearest, Eve appears at a bar in Grants Pass, Oregon, where she proceeds to turn everyone into hybrids monsters, provoking a mass feeding frenzy while she picks up a drink and watches. She is using the town as a test-bed to create a new type of monster which she wants to use to convert all of humanity. Several of her early experiments result in failure as the creatures die prematurely.

With Bobby and Castiel waiting outside, the brothers enter the diner in which Eve is hiding and discover that they are surrounded by hybrids and Eve herself confronts them in the guise of a waitress. Eve takes their weapons and detects Phoenix ash in the shotguns, so she hands them over to one her children and orders their immediate disposal. She then explains to the boys the reason she's on Earth and what she hopes to achieve.

To emphasise her point, she transforms into the likeness of Mary Winchester and tells them that every action she's taken has been because of "a mother's love". Eve goes on to explain that she had been satisfied with the balance between mankind and monsters, but since her children started being captured and tortured by Crowley, she had decided to rise up from Purgatory and retaliate. Dean is quick to point out that her arrival was pointless, as Crowley had already been dealt with months ago.

Eve is unconvinced, however, and informs them that Crowley is indeed alive and in hiding, but is still carrying out his search for Purgatory. She explains that Crowley is not after Purgatory itself, but rather the souls that dwell within it. Monster souls, like humans, are very powerful and are essential for fueling a war machine capable of untold power. For what purpose Crowley wants this power, she does not reveal. But to cripple his plans, Eve intends to turn the whole world into monsters so that she can "see how hot his hell burns when all your souls come to me".

Eve then informs them of the "perfect monster" she has created in Ryan Silver and of the mass-infection that they inadvertently helped spread. Her monsters bring Castiel and Bobby into the diner, and informs Castiel that he can't use his powers because there being halted by Eve due to her age. She offers Sam and Dean the chance to work for her in capturing Crowley. Dean refuses to work with a monster, however, and tells Eve to "bite me".

She obliges, but staggers back upon tasting phoenix ash in his blood, to which Dean reveals that he ingested some ash with a shot of whiskey. Eve chokes and burns from the inside before falling to the floor, dead. The monsters attack all at once, but with Eve no longer suppressing Castiel's powers he releases a devastating blast of white light that kills all the monsters in the diner.

Castiel then teleports them all to Joe and Ryan's uncle's house to stop the children, who have already murdered their uncle, only to discover both kids dead and traces of sulfur; the work of a demon. Castiel later returns to the diner and observes Eve's body and the corpses of her children before being met by Crowley, who expresses his dissatisfaction with having to clean up after the angel's messes.

Powers & Abilities

 * Immortality - It is impossible for her to die, unless Phoenix ashes are involved.
 * Invulnerability -
 * Flight - She is capable of flight.
 * Possession - It is required that she use a vessel, and that of a virgin.
 * Omnipotence - Eve has the power to create something out of nothing, which is a form of this. Creating new monsters is easy for her, with just a touch or a whisper, and it can be done. She has to go through a few iterations, what she calls beta tests, before she perfects her last monster, the Jefferson Starships.
 * Super-strength - Her strength is far superior to humans and every supernatural creature.
 * Super-speed - She can move at undetectable speeds.
 * Shapeshifting -
 * Magical Knowledge - Her overall knowledge is extensive, but is by no means omniscient.
 * Telepathy - Eve can speak to monsters through their minds and control them.
 * Super-senses - She possesses heightened senses.
 * Psychokinesis -
 * Regeneration - Enough though it's nearly impossible to harm her, if she does happen to get hurt, she can heal almost instantly.

Trivia

 * It's possible that she is (in-universe) the inspiration for Echidna who, in Greek mythology, was known as "The Mother of All Monsters". however she calls herself Eve, who in the Old Testament of the Bible was the first woman to have been created, as well as the first woman to have disobeyed God. This does not count Lilith, supposedly a woman created alongside Adam who later left him and went on to have children with demons, because she is not universally accepted in Abrahamic religions.
 * Despite taking a human vessel, her true visage, or a portion of it, can be revealed on a camera recording.
 * Even after her death, her body still produced new monster eggs.
 * It looks like her latest offspring were linked to her, as shown when Crowley chopped into her dead brain the offspring felt the pain.

Season 6

 * Like A Virgin
 * ...And Then There Were None
 * Mommy Dearest