Season 5 is the culmination of the original five-season plan, pitting Sam and Dean against the Apocalypse itself. Michael and Lucifer are destined to use the Winchester brothers as their vessels for a final confrontation that will determine the fate of Earth.
Creatures Introduced
The Four Horsemen of the Apocalypse were introduced as distinct characters: War, Famine, Pestilence, and Death. Death in particular became a significant recurring figure, portrayed as an ancient, calm, and immensely powerful being who predates God's creation. The Anti-Christ (Jesse Turner, a human child with reality-warping powers) appeared in a single episode. Pagan gods had their most significant episode in "Hammer of the Gods."
Major Lore Developments
The cosmic hierarchy was fully established: God exists but has left, angels are fighting among themselves, and the Apocalypse is as much an angelic plan as a demonic one. The Horsemen's rings were revealed to be the key to re-opening Lucifer's cage. Gabriel was revealed to be the Trickster and died confronting Lucifer. The season ended with Sam sacrificing himself to drag Lucifer back into the cage, a resolution built entirely on free will trumping destiny — the show's core thesis.