Season 12 introduced the British Men of Letters, a ruthlessly efficient international organization that sought to impose systematic monster elimination on the more free-form American hunting community.
Major Lore Developments
Mary Winchester was resurrected by the Darkness as a gift to Dean, creating new family dynamics. The British Men of Letters' advanced technology and systematic approach contrasted with American hunters' improvisational style. Lucifer's child — a Nephilim of unprecedented power — was conceived with the human Kelly Kline. The season explored the ethics of preemptive monster elimination versus case-by-case hunting. The season ended with the birth of Jack (the Nephilim), the death of Crowley, the death of Castiel (temporary), and Mary being trapped in an alternate universe with Lucifer.