At the end of Season 10.
So, Death was going to send Dean to another world where he'd live forever with the Mark of Cain, unable to hurt anyone, right?
And first Death said he could remove the Mark from Dean if Dean was willing to transfer it to someone else, to which Dean declined, not wanting anyone else to go through what he did, right?
Well, did it ever cross Dean's mind to have Death transfer the Mark from him to Rowena, then seal her in the other world? That would remove both the Mark and Rowena from the Winchesters' lives forever without releasing the Darkness.
I imagine that's how the episode would have ended if the writers had felt they'd satisfied their torture fetish for Sam and Dean enough, because that would have ended the series, unless Crowley stayed on as main villain of Season 11 because he'd gone back to the dark side (from being at least partially on the light side through Seasons 9 and 10).