Season 8 centered on the Men of Letters (a scholarly organization that catalogued supernatural knowledge), the Trials of God (tasks that would permanently seal Hell), and the ongoing tablet mythology.
Major Lore Developments
The Men of Letters bunker became the Winchesters' permanent base, providing access to an enormous archive of supernatural research. The Trials of God — three tasks that would seal the gates of Hell forever — drove the main plot: bathing in hellhound blood, rescuing an innocent soul from Hell, and curing a demon. Sam undertook the Trials but was stopped before completing them, as the process would have killed him. Naomi was introduced as an angel bureaucrat who had been controlling Castiel through brainwashing. Metatron, the Scribe of God, was revealed and began his plan to close Heaven.