WARNING: The following article contains spoilers for Season 15 of Supernatural.
Following Supernatural's resident witch queen, Rowena, nobly sacrificing herself to seal the rupture God caused between Hell and Earth, Sam and Dean were left bereft of not only an enemy-turned-friend but also their trump magical card. The episode "Golden Time," however, provided an unexpected solution to this problem. Supernatural's new witch supreme is none other than Sam Winchester.
Sam's ascension was predicated by the sudden reappearance of Eileen Leahy, a Deaf hunter who was ripped apart by a Hellhound in Season 12 thanks to Arthur Ketch (another of the Winchester's adversaries-turned-allies). Thanks to the rupture, Eileen -- like millions of other souls -- was unleashed from her subterranean prison. Once the damage was re-sealed, she found herself stuck between two awful choices: return to the torturous realm of demons or aimlessly wander Earth and eventually go mad; a fate that the similarly returned spirit of Kevin Tran warned would happen. Eileen sought the help of Sam and Dean, with the latter suggesting they cook up another Soul Catcher to house her spirit.
While this private jail cell wasn't ideal, Sam agreed to take Eileen to Rowena's apartment to source the materials the ritual requires. There, he stumbled on something even better: the resurrection spell that the ancient witch began working on after the Nephilim Jack "disappeared" Mary Winchester. Upon learning that Sam and Dean's mother was content in Heaven -- the best possible outcome for a hunter -- Rowena put the spell to one side but, spurred on to do some good after a spate of heavy losses, Sam excitedly got to work on completing it for use on Eileen instead, and was even more excited when a newly-revived Eileen appeared in front of him. Resurrection, as you'd expect, is no mean feat. In the world of Supernatural, only archangels and those on the level of God and Death can pull it off unaided. That's the level of magical competency Sam has just achieved.
Sure, you could argue that Rowena did most of the heavy lifting, but the fact that Sam had the know-how and skill required to finish the work of the world's most powerful witch shouldn't be underestimated. In fact, according to the mother of the trio of witches who try to ransack Rowena's apartment in the episode, the spell is so rarified it has a one-time use, as Billie (Death) would become wise to this "loophole" straight away. In "The Rupture," Rowena even claims that Sam "is the closest thing to a witch" the group has other than herself. And, as more proof of Sam's solo sorcery acumen, when he, Dean and Eileen face-off against the family coven, Sam whips up his own hex bag on the spot and uses it to deadly effect. While Sam won't be able to repeat Rowena's invented incantation, there's nothing to say he couldn't come up with one of his own should he continue to develop his burgeoning witchcraft career.
Though it's might seem like an unexpected development, Sam becoming Rowena's protégé hasn't come out of nowhere. It's revealed in "Golden Time" that Sam is the only person Rowena made immune to the anti-intruder curse that she placed on her home, which is as good as naming him the sole heir of her entire library of magical items and books. Rowena also forcibly made Sam the one to kill her -- as was divined by Billie -- which, in a kind of gruesome way, cemented the bond that had been slowly building up between them as Rowena's self-serving nature gave way to something more altruistic. (A certain section of the Supernatural fandom has even gone so far as to interpret their chemistry as having romantic potential.)
Even before Rowena came along, Sam has long had a greater affinity for lore and magic than Dean, the more action-oriented of the pair, who has labeled his younger brother a "walking encyclopedia of weirdness" in the past. As well as memorizing numerous Latin incantations for use in the field, we've seen Sam perform all kinds of high-level spells across Supernatural's 15 seasons, from summoning and banishing low-level gods; combusting witches; "curing" demons; creating special bullets for the Colt; using Sigils to downgrade Lucifer and even sealing interdimensional tears in reality. Really, resurrection magic is just the crowning achievement.
The unfortunate circumstances of Sam effectively becoming the team's Rowena stand-in shouldn't go unnoticed, though. With the exception of Castiel, none of the Winchester's allies have long life expectancies, but women are especially at risk of suffering grim fates, which we're only reminded of by Eileen's reentry. Rowena's loss isn't just a loss in-universe: next to Jody Mills and, formerly, Mary Winchester, she was the most prominent female character on the show, as well as the strongest. It's a real shame that she had to fall in order for Sam to rise, though -- as is always the case on Supernatural -- there is still some wiggle room for her to make a return.
Until that happens, Sam has little choice but to fully embrace his inherited title as Supernatural's new mightiest mage. After all, with Chuck (God) still gunning for them, the brothers really need some serious magical firepower.
Airing Thursdays at 8 p.m. ET/PT on The CW, the final season of Supernatural stars Jensen Ackles, Jared Padalecki, Misha Collins and Alexander Calvert.