Spoilers ahead for Supernatural season 15
Supernatural's final season has already clocked up a fairly significant death toll, but the epic demise of one major character has already been spoiled. Joining the Supernatural fold in season 10 as Crowley's (literal) witch of a mother, Rowena gradually transformed into a genuine friend to the Winchesters and developed a particular fondness for Sam, working with the hunter on several spells that would help save the universe. Unfortunately, it was revealed that the younger Winchester brother was actually fated to kill Rowena in a series of events that was impossible to avoid and, in season 15's third episode, this prophecy finally came to pass.
After God opened every door in hell during Supernatural's season 14 finale, deadly and disgruntled ghosts began swarming the Earth. The Winchesters, along with help from new demon friend Belphegor, manage to contain the spirits temporarily but it was Rowena who came up with a more permanent solution. Sacrificing her own life to save the world, Rowena was stabbed by Sam and then absorbed all of Hell's escaped souls into her own body before casting herself into the fiery rupture to ensure the day was saved. Rowena's death was a powerful moment in Supernatural's final season and the first major casualty in the inevitable upcoming bloodbath.
In "Our Father Who Aren't In Heaven," Rowena returns, only a handful of episodes after her apparent demise. After being sent to Hell, the witch apparently managed to jostle her way to the top of the food chain and has become the leader of the underworld, permitting her to wander around freely like any other character. Ruth Connell's character goes on to help Sam and Dean in their quest to find the archangel Michael, using the new demonic resources at her disposal.
It's extremely common for dead characters to return to Supernatural, and it could even be said that out of life's two certainties, only "taxes" seems to apply in the world of Sam and Dean. Rowena's return is just the latest in a long line of character comebacks. The problem is, Rowena's death was built up to feel like a permanent, satisfying, redemptive conclusion to her story. The long-term foreshadowing that Sam would have to kill his close friend made for a tragic slow-burn every time Rowena appeared on Supernatural, and an entire episode was dedicated to the sacrifice she made to save Earth, complete with epic music and a long, drawn-out walk to face destiny.
The death of Rowena has carried major consequences in subsequent episodes also. Sam's demeanor changed entirely as he mourned the death of a close ally and carried the guilt of striking the final blow, creating a bitterness that briefly bubbled beneath the surface. Sam Winchester also became Rowena's de facto successor when he gained access to her magic lair and found that he was the only person permitted to use its contents, going on to complete a half-finished spell the witch had left behind.
Bringing Rowena back so soon after her departure creates problems for Supernatural. Firstly, the character's return cheapens the long-term storytelling that built up Sam's destiny to kill Rowena for so long. This plot point seemed like a major factor, but the witch's immediate reappearance makes the prophecy seem inconsequential. Something similar can be said for the character's actual death scene from episode 3. The sequence was an emotional exit for such a popular character and since Supernatural is currently in its endgame, the death felt permanent. Now, an emotive and tragic episode is stripped of its gravity, and the solution to Chuck's opening of hell no longer has any price attached to it, since Rowena has, if anything, upgraded. Sam's grief and succession of Rowena's magical artifacts has also become less important with the character now back on the scene.
It could be argued that Rowena's return doesn't even progress Supernatural season 15's story. Given that the Winchesters are on a crusade against God himself, any self-respecting leader of Hell would surely have given them the help they required, since demons aren't exactly on good terms with the big man upstairs. Rowena's return, therefore, doesn't serve to advance the plot in a way that another figure couldn't have managed.
Supernatural season 15 returns in 2020 on The CW.