HOLD ON A TICK.
The first pic is when Cain shows Dean and Crowley the Mark of Cain. The second one is after he’s sent Crowley and Dean out of the house—after he’s given Dean the Mark.
Cain clearly still has the mark in the bottom pic, so the mark is something he can give to another without giving it up himself, and we know that it confers some sort of power, and allows the marked person to wield the First Blade.
We also know that Lucifer hand-picked the Knights of Hell, Cain being the first one, that Lucifer himself gave Cain the mark, and Cain then went on to train the Knights.
I have a very bad feeling, here, that the Mark is specifically related to Cain being a Knight of Hell, and that Dean, who refused to listen to Cain when he tried to point out the fine print of accepting the mark, may have just signed on as a new recruit.
This makes a lot of sense, particularly given all the hints/foreshadowing of Dean being possessed.
1. Abaddon commenting on his perfect vessel.
2. Possession in Slumber Party (together with Sam who has already been possessed this season. It would be a nice parallel.)
3. Dog Dean Afternoon with Dean being unable to control the canine side-effects of the spell.
Hell, in a way I would even add the Suzie hook-up as possession in the sense of Dean only being able to sleep with his fantasy porn star by acting as a character in a porn (hell he even switches languages) and his inability to see her as anything than a fantasy/object.
Basically, we have seen a lot of Dean’s character traits presented as marks of possession/obsession. Traits that have always been presented as endearing, quirky or positive before. This season is giving a big fuck you to a lot of them. Loyalty in Dog Dean Afternoon came with uncontrollable negative side-effects. Choosing Sammy over his life at the home for boys in Bad Boys was presented as detrimental with Sam even apologizing. Dean’s liberal, easy-going attitude towards sex made him look like a grade a asshole just this side of noncon in Rock and a Hard Place. His need to protect Sam turned into something that got other family killed in Holy Terror. His mantra of Free Will turned into making choices for others throughout the season, but particularly with Sam and Cas. Dean’s positives turned into negatives.
And don’t even get me started on how many torture references he has made or how often he has actually tortured: “lets Zero Dark Thirty his ass”, isolating Crowley, to name but a few.
In addition, the Mark not being transferred 100% would mirror the grace that stays behind in Vessels after angel possession nicely.
So, yeah. This is a scary, but very probable piece of speculation here
edit: it would also put Crowley’s comment of Dean being worthy in one hell of a scary context.