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 while rogue angels roam the Earth causing trouble as well. Castiel is now human and having to adjust to his life as a human 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 evil 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. Dean takes on the Mark of Cain despite being warned of terrible repercussions in order to be able to wield the Blade. Castiel begins a search of his own for Metatron, believing him to be the key to reversing the expulsion of the angels after healing Sam completely.
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. Dean takes on the Mark of Cain despite being warned of terrible repercussions in order to be able to wield the Blade. Castiel begins a search of his own for Metatron, believing him to be the key to reversing the expulsion of the angels after healing Sam completely.