Season synopsis
Contains spoilers for this season and earlier seasons.
After the angels fall, Sam is close to death from the trials and Castiel is human, having lost his grace. Dean lets an angel calling himself Ezekiel possess Sam so the angel can heal him, and keeps the arrangement secret. Ezekiel is actually Gadreel, an angel once imprisoned for failing to guard Eden. Castiel survives on Earth among the fallen angels, then takes the grace of another angel, Theo, to regain angelic power and rejoin the fight over Heaven.
Gadreel accepts Metatron's offer to help remake Heaven and uses Sam's body to carry out his orders. He kills Kevin Tran while Sam is unaware. With Crowley’s help, Sam learns the truth and ejects Gadreel, furious that Dean allowed the possession. Dean's own path turns darker when he accepts the Mark from Cain and tracks down the First Blade with Crowley. Dean kills Abaddon with the blade, but the Mark's influence grows as the angel conflict escalates and Metatron pursues control of Heaven.
Gadreel sacrifices himself to free Castiel from Metatron's prison, and Castiel destroys the angel tablet that powers Metatron's spell. Dean knocks Sam unconscious and confronts Metatron alone on Earth. Metatron stabs Dean with an angel blade just as Sam arrives, and Dean dies. Sam carries his body to the bunker, where Crowley places the First Blade in Dean's hand. The Mark revives Dean as a demon, ending the season with Sam's brother alive but transformed.