After learning of the state of Heaven, the Scribe of God Metatron took advantage of the Seraphim Castiel's desire to fix the damage he did to Heaven to trick him into helping him gather the ingredients for a spell by claiming that Castiel was actually doing the Trials to close the gates of Heaven. Metatron was able to successfully dupe Castiel into helping him and cast his spell, causing all of the angels to Fall to Earth. In the process, Metatron stole Castiel's grace as part of the spell, rendering him human.
Following the Fall, the angels started gathering into factions with many of the factions wanting Castiel dead for his role in the Fall. One faction in particular, led by Bartholomew hunted for Castiel but failed to catch him, leading to Castiel warding himself against angelic detection. After the angel Gadreel forced Dean to throw Castiel out of the Bunker, Castiel tried living a human life before being disturbed by the reminder of how the angels needed his help after an encounter with the Rit Zien Ephraim.
Sometime after his encounter with Ephraim, the former Seraphim Castiel hears of some angel killings and decides to investigate, wanting to do what he can to help if angels are slaughtering each other. While on the case, Castiel meets up with Sam and Dean who are investigating the same thing and he explains his reasoning for investigating the murders to them along with the fact that the angel kills were done with more violence than was needed. As the three drink at a nearby bar, Gadreel demands of Dean that Castiel leave while he's gone to get more beers and Dean has no choice but to comply, telling Castiel that the Trials messed Sam up badly and he has to do whatever it takes to save him, including making Castiel keep his distance from them. Castiel reluctantly complies with Dean's wishes and leaves.
Desperate for information, Castiel tries praying to the angels for help. Eventually, the neutral angel Muriel responds to his call and though reluctant once she realizes who he is, agrees to talk to him. While Muriel suggests that she could turn him in even if she believed his story about being duped by Metatron, Castiel refuses to believe that she would as they both want no part of the fanatics. Castiel explains that as he's warded and his grace is gone, he'd seem to the fanatics like just another human praying for help and they wouldn't care about him. Castiel asks Muriel about the blood feud between Bartholomew and another angel faction which she calls madness. Muriel tells Castiel that Malachi leads the opposing faction which shocks Castiel as he knows Malachi as an anarchist. Muriel explains that Malachi has amassed such a following that he's equals with Bartholomew and is in some ways even worse. Muriel further tells Castiel that the neutral angels are fewer and fewer as the two main factions are rounding up those angels who are neutral and they are being tortured and killed if they don't pledge loyalty to the factions. Muriel and Castiel recognize that the factions goals are to crush each other, overthrow Metatron and retake Heaven, but know that Heaven under either of faction would be Hell. At that moment, Theo and another angel arrive, having tracked down Muriel and capture her and Castiel.
Castiel and Muriel are taken to Malachi himself who is pleased with the fact that they captured Castiel and begins torturing him and Muriel. Castiel insists that he was an unwitting accomplice to Metatron's plans and doesn't know how Metatron's spell works or how to reverse it. However, Malachi refuses to believe that Castiel was duped and has Theo continue to torture him. When Castiel continues to refuse to give him the answers he wants, Malachi has Theo kill Muriel as a lesson to Castiel. Afterwards, Malachi blames the host of angels who died in the Fall on Castiel and names a few, including Ezekiel which draws Castiel's interest as Ezekiel had supposedly helped Dean heal Sam of the damage from the Trials. When Castiel still refuses to answer, Malachi leaves him to be tortured by Theo.
After Malachi leaves, Theo shocks Castiel by asking him to contact Metatron on Theo's behalf and ask Metatron to return him to Heaven. Theo explains he's willing to do whatever Metatron wants for him to be able to return to Heaven, explaining he'd thought Malachi was the answer but has come to realize he's insane instead. Theo tells Castiel that while he'd originally thought that Malachi could defeat Bartholomew, he's come to realize that no one will survive the coming war and wants to sit it out in Heaven and believes Castiel has the pull with Metatron to make it happen. Castiel pretends to agree that Theo would make a good soldier for Metatron and gets Theo to release him. Once he's freed, Castiel tells Theo he needs something before he contacts Metatron and then grabs a nearby angel blade and slices open Theo's throat, releasing his grace. Castiel absorbs the grace through his mouth and is restored to being an angel. Seeing Theo is still alive, though now human, Castiel uses his restored power to smite the former angel and fights his way out of Malachi's hideout, killing three more angels along the way.
Following his escape, Castiel calls Dean to inform him that Malachi is the leader of the other faction. Castiel tells Dean of his capture and that he did what he had to do to escape, becoming a "barbarian" in the process. To Dean's surprise, Castiel tells him that he got an angel's grace and is once again an angel himself though he's unsure of how powerful he is. Castiel insists that if the angels are going to war, he needs to do whatever it takes to be ready. Castiel asks Dean about how Dean had told him that Ezekiel had helped heal Sam and when Dean confirms it, Castiel tells him that Ezekiel died when the angels Fell, worrying Dean as Ezekiel was supposedly possessing Sam.
Castiel eventually returns to the Bunker to learn of Sam's kidnapping by Gadreel and his murder of Kevin Tran. Castiel helps free Sam from Gadreel's control then sets out to stop Metatron and bring the angels back to Heaven. During his journey, Castiel battles Bartholomew and kills him while Malachi is eventually killed by Gadreel, ending the angelic war between the two.
While Castiel is once again an angel after stealing Theo's grace, it slowly starts to burn out and will eventually kill him if not replenished. Castiel eventually replaces Theo's grace with Adina's before finally getting his own back.
- This battle occurs at the same time as the Battle between Bartholomew and Malachi and the Assassination of Kevin Tran.
- Castiel is once more restored to his status as an angel though he is no longer a Seraphim due to using Theo's grace.
- Despite making a big deal of the coming angelic war between Bartholomew and Malachi, Castiel only ever faces off with Bartholomew again and nothing of the war itself is ever seen or mentioned afterwards.
- This event features the last time Castiel smites another being until Season 13's Various & Sundry Villains.