Season 9 dealt with the aftermath of Metatron's spell casting all angels out of Heaven. Thousands of wingless, confused angels were stranded on Earth, creating chaos and new threats.
Creatures Introduced
The pishtaco (from Andean folklore) appeared. The Knight of Hell Abaddon returned as a major antagonist. Gadreel, the angel who failed to protect the Garden of Eden, possessed Sam at Dean's request to heal him. The Mark of Cain was introduced — an ancient curse that granted immense power but corrupted the bearer.
Major Lore Developments
Dean received the Mark of Cain from Cain himself (revealed to be alive and living as a beekeeper) and used the First Blade to kill Abaddon. The Mark's corrupting influence became the season's central tension. Metatron positioned himself as the new God of Heaven, rewriting reality to fit his narrative. The season ended with Dean's death and resurrection as a demon — the Mark of Cain transforming him into a Knight of Hell.