Mark of Cain
The Mark of Cain is a brand placed on Cain by Lucifer. 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 if the person is worthy of it, i.e. a killer.
Episodes
9.11 First Born
Knowing that Dean wants the First Blade to kill Abaddon, and after seeing him kill a number of demons, Cain decides that Dean is "worthy" of carrying the Mark in order to wield the blade. Cain transfers the mark to Dean, where it appears on his right forearm. According to Cain, taking on the mark comes with a great cost and burden.
Mark of Cain in Lore
In the Abrahmic religions it is said that after killing Abel, God cursed Cain, saying that he would be forever separated from his family and become a nomad. The mark of Cain was said to be placed on Cain by God so "Therefore whoever kills Cain, vengeance will be taken on him sevenfold. And the Lord appointed a sign for Cain, so that no one finding him would slay him." (Genesis 4-14)
