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The Mark of Cain was a lock designed by [[God]] for [[the Darkness]]. He originally gave it to [[Lucifer]] who passed it onto [[Cain]] and from there to Dean. It was the first [[curse]] in existence and the source of power for [[the First Blade]].<ref name="ten17">[[10.17 Inside Man]]</ref> [[Amara ]] being the original Mark, it appears on her body shoulder as a birth mark or a scar unlike with Cain and Dean, where it appeared on their forearms as a brand.

According to [[Death]], before there was life , the Earth, or even God and the Earth[[Archangels]], there was [[the The Darkness]]. [[God]] and His [[archangels]] defeated the Darkness and locked it away rather than destroy her to maintain [[The Cosmic Balance]]. God created the Mark as the lock and key on the Darkness' cage and if it is destroyed, the Darkness will be released. God gave the Mark to his most trusted lieutenant, [[Lucifer]], but the Mark corrupted Lucifer and made him jealous of humanity which led to his imprisonment in [[The Cage]]. Lucifer eventually passed it onto the Mark to [[Cain]] as part of his deal to save Abelfrom Lucifer's influence.

After Cain was given the Mark, he begins to realize what kind of toll bearing the Mark will take after he killed Abel, and rather than be controlled by his murderous compulsion, he attempts to kill himself with the First Blade. However the Mark would not let him diehis soul depart to the afterlife. After Cain's soul was twisted and corrupted by the Mark's dark, influential, primal energies, and he was reborn as a [[Knight of Hell]], the very first of his kind. He goes on to train and lead the Knights of Hellunder Lucifer's command, as they wrecked havoc on Earthfor centuries as the harbingers of chaos and darkness.<ref name="nine11">[[9.11 First Born]]</ref> .

After centuries of murder and mayhem, he fell in lovewith [[Colette Mullen]]. The Knights took retribution and took Colette, so Cain used the First Blade and despite its energy drawn from the woman he loved being killed by Mark to slaughter all the Knightsexcept for one; [[Abaddon]]. Despite Colette's death by Abaddon's hands, he Cain manages to cease killing , keeping his final promise to her, after he disposed of the First Bladein [http://en.wikipedia.org/wiki/Mariana_Trench The Mariana Trench].

The Mark imbues its owner with great strength and an almost insatiable desire to kill. This strength is enough to overpower an entire nest of [[vampires]] and even [[Castiel|an angel]]. It is the source of [[the First Blade]]'s power. The Mark and the Blade work together, and without it the First Blade is rendered useless. The Mark can also be transferred to someone else if the person is worthy of it; a killer like Cain. While the Mark is a corrupting influence, [[God]] revealed to Lucifer that the Mark does not change the bearer, but merely amplifies and intensifies what is already there. Which in Lucifer's case was his dislike of humanity.