Supernatural (Season 9)

 In the first half of the ninth season, the angels have fallen to Earth and two factions are warring against each other while hunting Castiel, rogue angels roam the Earth causing trouble as well. Castiel is now human and having to adjust to his new life while Crowley is Sam and Dean's prisoner and Abaddon works to take over Hell. Sam is left near-death from the Trials of God and Dean is forced to let an angel claiming to be Ezekiel possess Sam to heal him from the inside. While dealing with the various threats that crop up, Dean must hide Ezekiel's presence from Sam so he won't expel him and die while also keeping Castiel away at Ezekiel's demand. The brothers must also search for a way to return the angels to Heaven. Eventually it is revealed that Ezekiel is actually the angel Gadreel who let Lucifer into the Garden of Eden. Gadreel murders Kevin Tran and escapes while Castiel regains his powers. With Crowley's help, Sam expels Gadreel, but the experience causes Sam and Dean to split up while letting Crowley go as part of the deal Dean made for him to save Sam.

In the second half, Dean begins searching for both Gadreel for revenge and the First Blade to kill Abaddon with the help of Crowley with whom he forms a reluctant alliance as Abaddon has begun attempting to take over Hell and is "much worse" than Crowley. Despite being warned of terrible repercussions, Dean takes on the Mark of Cain in order to be able to wield the First Blade. After healing Sam completely, Castiel begins a search of his own for Metatron, believing him to be the key to reversing the expulsion of the angels. Metatron begins trying to unite all the angels under his rule while Castiel leads other angels against him to retake Heaven. Eventually Dean kills Abaddon and Castiel defeats Metatron with the help of a repentant Gadreel, but Metatron kills Dean, causing him to become a demon himself due to the influence of the Mark of Cain.