Season 7 introduced the Leviathans — God's first and most dangerous creation — as the primary antagonists. Their corporate infiltration of American society gave the season a different flavor from previous arcs.
Creatures Introduced
Leviathans were the major addition, along with vetalas, the shojo (a Japanese alcohol spirit), and Amazons. The season also featured the return of familiar creatures in new contexts, including ghosts, werewolves, and vampires. Borax was discovered as the Leviathan weakness. The concept of the Word of God tablets — stone tablets containing divine instructions for various supernatural threats — was introduced via the prophet Kevin Tran.
Major Lore Developments
Bobby Singer was killed by Dick Roman and spent several episodes as a ghost before being salt-and-burned. His death was a watershed moment for the show. The Leviathan plan to domesticate humanity through food additives was revealed. Dick Roman was killed using the bone-of-a-righteous-mortal weapon, but Dean and Castiel were pulled into Purgatory as a consequence. The season established that Purgatory was a harsh, combat-driven afterlife for monsters and that escaping it was possible but extremely difficult.