|“||Samuel. I hope y'all are having a pleasant day. Its come to my attention that you boys have something that belongs to me and I'd like it back. (...) And I'm gonna give you one chance to return him to me, no harm, no foul. (...) Do not hang up on me! Gabriel is of no use to you in his current condition. Should you choose to resist me I will have no choice but to take him by force! I will reduce you and that sad little bunker of yours to ashes! You got ten minutes to decide. Now you can hang up.||”|
|“||Your warding wasn't designed for the likes of me Samuel. I've come to claim what's mine.||”|
|“||Oh, by the way. I always hated your dumbass suit.||”|
|“||It was a demon incursion led by Asmodeus. (...) Yeah, and, and, and Gabriel killed him.||”|
After faking his death during the Apocalypse, the archangel Gabriel turns to the Norse god Loki and his children for protection. Though they originally agree, the gods eventually betray Gabriel, bind Gabriel and sell him to the Prince of Hell Asmodeus in revenge for Gabriel's role in the god Odin's death. Over the next eight years, Gabriel is extensively tortured by Asmodeus who feeds on Gabriel's grace in order to become more powerful.
In 2018, in response to the return and escape of Lucifer and the growing threat of the Apocalypse World Michael, Asmodues obtains the archangel blade, the one weapon that can kill an archangel in the hands of another archangel. While showing his new acquisition to mercenary hunter and former British Men of Letters operative Arthur Ketch, Asmodeus introduces him to the captive Gabriel.
Following Arthur's impudence in dealing with him, Asmodeus brutally beats Arthur in retaliation, leading to Arthur rescuing Gabriel and stealing the archangel blade and Asmodeus' store of Gabriel's extracted grace. Arthur delivers all three to the Winchesters at the Men of Letters bunker in exchange for sanctuary at the bunker. Dean agrees and he and Arthur subsequently depart through a rift to Apocalypse World on a mission to rescue Mary Winchester and the Nephilim Jack, leaving Sam behind to deal with the injured and traumatized Gabriel. Over the course of the next day, Sam and Castiel attempt to treat Gabriel, giving him the last of Asmodeus' store of Gabriel's extracted grace. At the same time, Asmodeus attempts to seek out Gabriel in order to recapture him.
After finally sensing that Gabriel is in the Men of Letters bunker, Asmodeus calls Sam who is shocked to hear from him. Asmodeus tells Sam that he knows they have something that belongs to him and he wants it back. Asmodeus offers Sam the chance to return Gabriel to him "no harm, no foul," but Sam goes to hang up on him. Asmodeus angrily tells Sam that Gabriel is of no use to Sam in his current condition and if Gabriel isn't turned over to him in ten minutes, he will take Gabriel back by force and destroy Sam and the bunker before hanging up on the worried Sam, Castiel and Gabriel.
Shortly afterwards, Sam enters Gabriel's room where Gabriel is rubbing his temples while Castiel watches. Sam tells Castiel that he has done what he can to help with the bunker's warding, but isn't sure if it will be enough. Suddenly, the lights flicker and go out with red warning lights replacing them alongside sirens. In the corridor outside of Gabriel's room, Sam and Castiel find the bunker's warding glowing and flickering on the walls as well. As the two make their way to the bunker's library armed with angel blades, the warding ultimately flickers out as it loses its power.
In the library, Sam and Castiel are suddenly ambushed by two demons. Sam quickly manages to throw his attacker on the map table and kill him with his angel blade, but is tackled to the ground by another demon. The demon and Sam struggle for control of an angel blade as the demon tries to stab Sam and Sam tries to prevent him from succeeding. At the same time, Castiel fights with his attacker who is also armed with an angel blade. Castiel overpowers the demon and knocks him out as Sam' manages to kick his attacker away. Before the demon attack again, he is grabbed by Castiel who slams the demon into the bunker computer and smites him.
As the two recover from the fight, Asmodeus suddenly appears and flings Sam and Castiel against the bunker's machinery with telekinesis. Asmodeus explains that the bunker's warding "wasn't designed for the likes of me" and that he is there to claim what is his. As Sam and Castiel watch helplessly, two more demons lead the captive Gabriel in. The pleased Asmodeus tells the terrified Gabriel that he has missed him and will have to punish Gabriel "rather severely" for his actions. As Gabriel is led out, Asmodeus turns his attention to Sam and Castiel, inflicting excruciating pain on them.
Hearing his friends in danger causes Gabriel to suddenly fight back, flinging one demon down the stairs and another over the balcony with just a shove, knocking them both out. As Gabriel comes to the edge of the balcony, Asmodeus angrily confronts him on his actions, stating that he has broken Gabriel and Gabriel is too weak to stand up to him. Gabriel heals his injuries instantly and displays his wings in a show of power.
Faced with a recovered Gabriel, Asmodeus throws an energy blast at Gabriel who effortlessly deflects it. Stating that he's not too weak anymore, Gabriel comments on how much he has always hated Asmodeus' suit before igniting Asmodeus with a hand gesture. Screaming, the last Prince of Hell and his vessel are incinerated by the enraged archangel.
In the aftermath of Asmodeus' death, Sam, Castiel and Gabriel kill the three unconscious demons, securing the bunker and ending the demon incursion.
After the end of the incursion, Sam and Castiel fill Gabriel in on the threat from Apocalypse World and Michael. However, Gabriel refuses to help them with the fight and departs once more. When Dean returns through the rift, Sam and Castiel explain the incursion to him and how Gabriel killed Asmodeus. With Gabriel gone, the Winchesters are left without the archangel grace they need to reopen the rift to Apocalypse World.
Following his rescue and the death of Asmodeus, Gabriel sets out to get revenge on Loki and his family for selling him to the Prince of Hell in the first place. Weakened by using most of his recovered grace to kill Asmodeus and leave afterwards, Gabriel is forced to use specially-crafted wooden swords to fight the gods with the help of the Winchesters. After killing Loki, Gabriel agrees to join the effort to defeat Michael's upcoming invasion.
- The death of Asmodeus marks the extinction of the Princes of Hell.
- While the three unconscious Bunker Incursion Demons are not killed on-screen, four of the bodies of the demons are seen strewn about when Sam and Castiel explain to Gabriel what is going on.
- Gabriel killing Asmodeus is the first time Gabriel kills someone on-screen.
- In Unfinished Business, Gabriel reveals that he used up most of his grace killing Asmodeus and leaving the Men of Letters bunker. As a result, Gabriel is forced to resort to weapons to kill Loki and his family and subsequently can't come up with the grace needed for the Demon Tablet Alternate Reality Rift Opening Ritual.