She was portrayed by Ali Ahn.
Dagon is the only female among the Princes of Hell, the first generation of demons after Lilith, created by Lucifer himself.
They acted as generals of Hell until Lucifer was imprisoned. Despite being the rightful heirs to Lucifer's throne, Dagon and her brothers Ramiel and Asmodeus opted to retire on earth with only Azazel assuming rule over Hell to continue their father's work. It is implied by Lucifer that at some point in the past, she had somehow failed him.
Ramiel tells the Winchesters that Dagon has taken an interest in Lucifer's son shortly before they kill him.
Dagon appears as Kelly Kline is attacked by two angels and causes both angels to combust. Dagon takes Kelly to a warehouse where she tells Kelly that her child is a boy and sees him as the best hope for everyone. Dagon offers to protect Kelly and the two leave together. As they leave, Lucifer senses that Dagon is with Kelly and smiles and calls out Dagon's name.
Castiel investigates the angels Dagon killed and finds video footage of it. Castiel recognizes Dagon and later calls the Winchesters to warn them about her. Castiel tells them all he knows of Dagon is stories and rumors about her psychotic savagery.
Dagon receives an unexpected message from Lucifer who wants to make sure she is protecting Kelly and the baby well. Dagon confirms that she is doing her best, to which Lucifer warns her not to disappoint him again. By morning, Dagon is ready to relocate Kelly to a new hideout, but Kelly refuses and wants her baby checked out because she is experiencing weird pain in her . Dagon reluctantly takes Kelly to a doctor who examines Kelly and initially says everything is fine. When Dr. Turner notices something off on the monitor, Dagon uses her powers to make him ignore it and reassure Kelly once more.
To cover their tracks and erase Kelly's, Dagon orders Dermott Culp to dispose of the doctor. Having previously ordered Dermott to burn down the warehouse she and Kelly were once at, hunter Eileen Leahy decided to follow Dermott and manages to kill him. While Dagon is out buying suppliments for Kelly recommended by Dr. Turner, Kelly is lured away by Eileen's friends, Sam and Dean Winchester. They try to take her back to their place but Dagon finds them. She teleports Kelly back to their hideout.
Dagon believes Kelly cannot be trusted so she handcuffs her to the bedpost. When Kelly argues that she was simply worried about her baby, Dagon tells her that the baby is indeed fine, but Kelly herself is going to die because birthing a Nephilim is always fatal; it is why Kelly is suffering from small pains, which are bound to increase in intensity.
Dean leaves Castiel a voicemail about their inability to kill Dagon even while wielding the Colt. Sam also searches through the many books the Men of Letters have on demons for information on her, but tells Dean that the Men of Letters have very little on Dagon and the Princes of Hell and nothing useful to them. Dagon continues to forcefully take care of Kelly, having been promised by Lucifer that the two of them and his son would rule the universe together. Dagon excitedly describes the destruction that the child will wreak upon his birth and leaves Kelly to clean up.
Upon Dagon's return, she finds the bathtub filled with blood and Kelly reveals that she committed suicide but her child resurrected her. Kelly insists to Dagon that the child is good and he saved her, but Dagon believes the child only saved his receptacle. Later, Dagon watches TV only to have three angels led by Castiel arrive, having tracked the pulse of power from Kelly's resurrection. Dagon is able to avoid an attack by Castiel with the Colt and kills Hozai and subdues Kelvin, but discovers that Castiel has escaped with Kelly while she took on the other two angels. Dagon informs an enraged Lucifer of her failure and Lucifer threatens her with eternal torment if she fails to recover his child. Dagon tortures Kelvin who eventually reveals that Castiel will likely go to Heaven's Portal. Dagon kills Kelvin and travels to the portal where she waits for several hours.
Finally, Castiel arrives with Kelly to take her through the portal to Heaven which will kill both Kelly and the child. As Joshua appears from the portal, Dagon kills him and takes on Castiel who proves to be no match for her. As Dagon prepares to kill Castiel, she is shot several times by Sam who arrives with Dean in Castiel's truck. Dagon shrugs off the attack, breaks Dean's arm and destroys the Colt.
As Kelly and Castiel face off with Dagon hand-in-hand, Lucifer's child empowers Castiel, enabling him to effortlessly grab Dagon's arm and render her powerless. Telling Dagon to "call it a miracle", Castiel sets Dagon ablaze with the child's help, burning Dagon to ash and killing her.