Season 10 focused on the consequences of Dean bearing the Mark of Cain and the increasingly desperate efforts to remove it.
Major Lore Developments
Demon Dean was cured early in the season using the demon-curing ritual from the Trials. The Mark of Cain's true nature was explored — it was not just a weapon or a curse but a lock, the original seal placed by God to bind the Darkness (Amara), God's sister. Removing the Mark would unleash the Darkness. The Book of the Damned, a powerful and dangerous spellbook, was introduced as a potential source for removing the Mark. Rowena, Crowley's mother and a powerful centuries-old witch, became a recurring character. The season ended with the Mark being removed by Rowena's spell, unleashing the Darkness.