<P> Faced with the threat of the Darkness--revealed over the course of the series to be God's' sister', sacrificed to create the world--Castiel agrees to act as Lucifer's vessel when Lucifer claims to be the only one with the power to defeat the Darkness, since God is absent and all other archangels are dead or insane . After killing Rowena, apparently the only person capable of re-opening the Cage, Lucifer spends some time re-establishing his power base in Hell while acting as Castiel to interact with the Winchesters and follow their research into Amara, but his true identity is revealed after the Winchesters attempt to travel back in time to recover a Hand of God from a submarine before it sank in 1943 . Castiel is able to regain control long enough to explain the situation to Sam and stop himself killing his friend, Dean subsequently banishing Lucifer and vowing to find a way to free Castiel from his control . Crowley, forced to act as Lucifer's slave, is forced on the run after he fails to kill Lucifer with another salvaged Hand of God . The Winchesters manage to get through to Castiel when preparing for a new confrontation with Amara, but Castiel resolves to remain as Lucifer's Vessel until her defeat, only for the Hand of God to fail against Amara's power . With Castiel / Lucifer now Amara's prisoner, she decides to torture an archangel to try and lure God out, leaving the Winchesters to speculate that Lucifer couldn't use the Hand of God due to the fact that he is a fallen archangel . Lucifer is eventually rescued when God returns--revealed to be Chuck, the author who wrote the Winchester Gospels--with God / Chuck healing Lucifer and apologising for his past actions . Although Lucifer is apparently killed in the later confrontation with Amara, Castiel is healed and returned to control of his body, although he is then banished when a member of the London branch of the Men of Letters attacks the bunker to punish Sam's past actions . </P> <P> Castiel is able to collect himself in time to return to the bunker and help Dean and the resurrected Mary Winchester find and rescue Sam . However, upon learning that Lucifer has escaped destruction, now jumping through vessels that rapidly burn out due to his weakened state making it harder for him to maintain them, Castiel sets out to find Lucifer, accompanied by Crowley, although their first attempt simply sees Rowena banish Lucifer's new vessel to the bottom of the ocean without actually managing to kill him . Lucifer eventually takes the President of the United States as his vessel, and conceives a child with one of the president's staff, but he is banished from this vessel thanks to the Winchesters and Crowley . After helping the Winchesters fake their deaths to escape the secret service, Castiel attempts to find Lucifer's child, but is convinced to spare the unborn child by his mother, Kelly, who is certain that the baby will be good . When Lucifer returns - Crowley faked banishing Lucifer when in reality he attempted to trap Lucifer in his old vessel and control him - the Winchesters are able to trap Lucifer in an alternate universe where the apocalypse took place as Dean and Sam were never born, but Lucifer kills Crowley and Castiel and takes Mary to the other world with him . </P> <P> Although Castiel is confirmed to be dead, Jack, Lucifer's son, who regards Castiel as his guardian, is able to make Castiel' wake up' in the Empty, the place where angels and demons go when they die, and is eventually returned to life when the entity that controls the Empty finds him annoying now that he is the first being to awake in that place . However, he is subsequently captured along with Lucifer when Lucifer returns to this world to warn them about the Michael of the parallel universe, who seeks to rule this world as well . </P> <P> Castiel typically displays very little emotion and always exhibits an extremely somber disposition . Although it has been suggested angels possibly do not possess the ability to truly feel emotion, Castiel frequently exhibits affection towards Dean . This attachment and growing ability to feel even causes him to be "demoted", as his superiors fear emotions are clouding his judgment . Throughout his appearances in the fourth season, he also seems to, at the very least, come close to expressing regret, hesitance, and anger several times, and has once quietly laughed at a joke Dean made in "It's the Great Pumpkin, Sam Winchester". Actor Misha Collins feels, through Dean, "(Castiel is) learning a little bit about humanity and re-learning something about (his) own humanity from that exposure . So...there's a humanizing of Castiel going on ." Castiel begins having an internal struggle between what is right and what is wrong, and whether or not to obey orders from Heaven . Collins feels this is because interactions with Sam and Dean have made the angel "uncertain" and "more fragile". Collins believes Castiel envies Dean's decisiveness and desires to emulate him . </P>

Who brought castiel back to life in season 5