Dagon (Ali Ahn) was a recurring villainess from Season Twelve of Supernatural. She is a Princess of Hell; the only female among the group of demons created by Lucifer.
Family Feud[]
With the exception of Azazel (a recurring big bad from the first two seasons), Dagon and the rest of the group remained in seclusion until the events of the episode, "Stuck in the Middle (With You)," when Ramiel revealed that Dagon was keeping close watch on Kelly Kline, who became pregnant with Lucifer's baby. Dagon's first official appearance came in the following episode, "Family Feud," where she killed a pair of angels who were after Kelly and her nephilim (half angel/half human) baby. She later took Kelly to a warehouse and later manipulated her into believing that the angels and the Winchesters were the true bad guys, and that her interests were genuine.
The British Invasion[]
In the episode, "The British Invasion," Kelly demanded to Dagon that she sees a doctor due to her weird pains. After refusing at first, Dagon took Kelly to Dr. Turner, and uses her demonic powers to coerce him into telling Kelly that her baby is fine. Later on, Dagon ordered her demonic henchman to kill Turner and erase any evidence of Kelly's visit. The demon is killed by hunter Eileen Leahy; after which Kelly is taken by Dean Winchester. Dagon catches up to the Winchesters and engaged in a battle between them and the rest of the protagonists, during which Eileen fired the Colt at Dagon. The demoness teleported away with Kelly, dodging the bullet, which fatally hit British Men of Letters member Renny Rawlings.
Dagon shows her true colors after she handcuffs Kelly to the bed, and reveals that the while the baby was fine, she won't be. She revealed that giving birth to a nephilim is fatal, which is why she's experiencing the pains.
The Future[]
Dagon's true goal was revealed in the episode, "The Future," as she had intentions of ruling the universe with Lucifer and the baby, per Lucifer's promise. She was forcefully tending to Kelly, and was disgusted over Kelly slitting her wrists in the bathtub, only for the baby to resurrect her with his powers. She was encountered by Castiel and a group of angels, and again avoided being killed by the Colt; but she lost Kelly to Castiel. Dagon tortured Kelvin into giving away Kelly and Castiel's location, and she tracked them and the Winchesters to the sandbox--the doorway to Heaven. During the battle, the villainess took down the brothers, and even destroyed the Colt.
At that moment, Kelly and Castiel stood together hand-in-hand, and their contact allowed the baby's power to be transferred into Castiel. The power allowed Castiel to subdue Dagon, and he later set Dagon ablaze and incinerated her.
Supernatural 12x19 Castiel kills Dagon