User blog comment:Twilight Despair 5/A (Cure) or whatever for the Mark of Cain/@comment-103.36.47.150-20150515050911

Well my theory (according to the promo of S10E23) is that it could either mean that it's Lucifer who must be killed to get rid of the curse, which is the task given to Dean by Death since he gave him his Scythe OR that the one who cursed Cain wasn't Lucifer but Death, and the task is to either kill Death and for Dean to become the new Death or Death will remove the Mark for him on one deadly condition.