Season 13 explored alternate universes — specifically "Apocalypse World," a parallel Earth where Sam and Dean were never born and the Apocalypse was never averted, leaving Michael and Lucifer's war to devastate the planet.
Major Lore Developments
Jack, the Nephilim son of Lucifer, grew rapidly and grappled with his immense power and his desire to be good despite his heritage. The Apocalypse World introduced alternate versions of familiar characters, including an alternate (and villainous) Michael. The season dealt with interdimensional travel, the nature of parallel realities, and whether fate is truly fixed or varies across universes. Gabriel was revealed to be alive (having faked his death in Season 5) but was eventually killed for real. The season ended with Apocalypse World Michael possessing Dean and crossing into the main universe.