While possessing President of the United States Jefferson Rooney, the Fallen archangel Lucifer conceived a Nephilim with Kelly Kline. Shortly afterwards, Lucifer was defeated and apparently locked away once more.
While hunters and angels sought out Lucifer's son Jack to kill him before he could be born, the Prince of Hell Dagon took an interest in the child and began protecting him and Kelly on Lucifer's behalf.
After discovering that Dagon is protecting Kelly and her child, the Seraphim Castiel is offered a pardon by the angel Joshua through Kelvin if he helps kill the child. Castiel, Kelvin and Hozai, armed with the Colt stolen by Castiel from the Winchesters, attempt to kill Dagon and Kelly. However, Dagon kills Hozai and captures Kelvin while Castiel is unable to bring himself to kill Kelly and instead flees with her. After extensive torture, Dagon learns from Kelvin that Castiel will likely head for the Heavenly Portal before killing him.
After fleeing with Kelly, Castiel is found by the Winchesters who bring forth a plan where they remove the child's grace, effectively rendering him human and a non-threat. At the same time, the angels wish Castiel to take Kelly through the portal to Heaven which will kill both her and her child. After the child has a premonition of a confrontation between Castiel and Dagon at the portal, he has Kelly steal the Impala and bring Castiel there.
At the Heavenly Portal, Kelly insists on going through with the plan, telling Castiel that as long as he's there, she knows that its going to be okay. The portal opens and Joshua emerges to greet Kelly and Castiel. As Joshua steps forward to reassure Kelly, the Prince of Hell Dagon teleports in behind him and combusts Joshua and his vessel into dust. Dagon jokes that they would've succeeded if she hadn't arrived at the last minute, then admits she got Kelvin to tell her where they were "ages ago" before killing him, allowing her to wait for them for hours.
As Dagon advances towards Kelly, Castiel draws his angel blade and steps in front of her protectively, amusing Dagon as he no longer has the Colt to use against her. Castiel tries to attack Dagon with his angel blade, but Dagon is far stronger and easily disarms Castiel before beating him to the ground. Dagon taunts Kelly about how she thought Castiel was going to protect her before lifting Castiel off the ground in perpetration to combust him.
As Dagon goes to kill Castiel, Sam and Dean Winchester arrive to help in Castiel's truck. Before Dagon can combust Castiel, Sam shoots her several times in the back. Sam's bullets have no effect on Dagon, but she drops Castiel to face off with the Winchesters instead. As Sam reloads, Dagon hits him with such force he crushes a nearby park bench as he falls. Dean then attempts to shoot Dagon with the Colt, but she teleports behind him, breaks Dean's arm and knocks Dean to the ground. Dagon declares that its time to take the Colt "off the board" and heats the gun up, causing the barrel to fall off and severely damaging the legendary gun to Dean's horror.
Heavily injured, Castiel makes his way to Kelly to Dagon's amusement and orders Kelly to run, but she refuses and takes Castiel's hand instead. As Dagon approaches to kill Castiel, Lucifer's son empowers Castiel through the connection between his mother's and Castiel's hands. As Dagon moves to combust Castiel, he grabs her arm, effortlessly restraining Dagon now that his strength has been enhanced by the Nephilim. As Dagon looks on in shock, Castiel negates her powers while the child heals his injuries. Telling the shocked Dagon to "call it a miracle", Castiel sets Dagon ablaze. Screaming in agony, Dagon and her vessel are incinerated, killing the Prince of Hell.
In the aftermath, Castiel tells the shocked Winchesters that it was him but also the Nephilim that killed Dagon. Castiel heals Dean's broken arm and thanks the Winchesters for coming to fight for them. Castiel states that he now knows that the child must be born with all of his power intact and he has "faith" in him. When the Winchesters refuse to let him leave with the child, Castiel renders them unconscious and departs in his truck with Kelly Kline. As they depart, Castiel reveals that while the child was empowering him, he showed Castiel "the future."
After returning to the Men of Letters bunker, Sam determines that the Colt was severely damaged by Dagon and may be beyond his ability to repair. The Nephilim, later named Jack by Kelly, is eventually born with the Winchesters, Castiel and Crowley fighting to keep Lucifer from getting his hands on him.
- This fight marks the third Prince of Hell killed by a member of Team Free Will. It is the first time a Prince of Hell is killed by someone other than one of the Winchesters however.