Using the demon cure process to try to turn his brother back,
In a future God shows Sam where the Winchesters and Castiel succeed in locking God away, Sam is eventually turned into a vampire on a hunt gone wrong. During a final confrontation with several hunters, including both Jody Mills and Bobby Singer, Sam is shot in the chest by Jody with a dead man's blood bullet.
Sam: Let Dean Become A Vampire
If not for the cure, Sam was risking either Dean be stuck as a vampire for a lifetime or killed to avoid that very fate. Sam with a soul never would've let it happen, but Sam still did this, making it one of the worst things he's done to Dean.
Sam and Dean investigate a series of missing girls in Limestone, Illinois, all of whom were Twilight fans. They realize the girls are being preyed upon by vampires and stake out a local bar to find the culprits. While there, Dean is turned by a vampire named Boris while Sam watches and lets it happen.
That woman, now vampire, is Jenny – who returns 15 years later in Supernatural's series finale as the leader of the nest of vampires who kill Dean Winchester.
Rather than some macho-heroic death or some world-ending mission (he'd been on plenty of those before, and even died in a few), Dean was impaled while fighting a masked vampire.
Nightsky: John was bitten by a vampire but, consistent with Supernatural lore, he wasn't transformed because he didn't ingest vampire blood or feed on human blood.
Dean forgives Cass, and they reconcile. However, Castiel admits that, technically, he is human, having depleted his "batteries" during his suicide mission.
Unknown to Sam, the ritual makes Dean human enough that he is able to escape and he stalks Sam throughout the bunker. However, Castiel finally arrives and subdues Dean. They are able to successfully complete the ritual and Dean is returned to being human, but retains the Mark of Cain.
Dean later briefly possessed a pair of Black Eyes while he was a Demon. Since separating from Sam, his appearance has become more scruffy and unshaven; his hair has also grown out a bit. In Let the Good Times Roll, Dean gains a pair of Angel's Wings due to his status as Apocalypse World Michael's current vessel.
Lydia is an Amazon who hooked up with Dean Winchester at a bar and the two made love in bed. She was impregnated by the older Winchester and in just a few hours, she was pregnant with her unborn child.
After all, despite his intentions, Sam did the unthinkable. He betrayed him for a demon. Dean tried to see past it and focus on task but the hurt was too deep and the betrayal too raw. He couldn't forgive.
Sam has all the advanced abilities of a vampire elder, but he seems to possess even stronger abilities than a typical vampire elder as he is the last elder chosen by the Dark One.
Sam is pregnant with Dean's triplet sons. Although he is only 7 months his belly looks like he is about to pop. Sam has a hard time moving around with the weight in his belly and is almost stuck in the bed till he gives birth.
By drinking demon blood Sam's power become stronger, to the point where he can outright kill demons and even torture them while in their human hosts; by drinking at least three gallons of demon blood, he can kill demons just by concentrating and willing it to happen.
As for Dean, he and all vampires are taken over by visions from the Alpha vamp. This is Boris' chance to set the vampires in the nest on Dean, but Dean is able to overpower them all. With Boris' blood, Dean is cured. However, he's also overcome by visions from when he was a vampire.
Fitting: Ended Up In Heaven
Most fans know that whenever a person dies, they either go to Heaven or Hell. Because Dean died several times in the show, it was only fitting that he not only died in the show's finale but that he was rewarded for his job as a hunter by going to Heaven.
Season 3. While trying to cope with his impending death and make the best of his final year, Dean begins to start hunting the freed Demons. Meeting a seemingly helpful demon Ruby, Dean learns that he too may become a demon himself if he burns in Hell too long.
Dean is resurrected as a demon at the end of season 9. After Dean accepts the Mark of Cain from Cain himself in season 9, it begins changing him, granting him supernatural powers, ramping up his bloodlust and making him addicted to using the First Blade.
However, his affection for Castiel ultimately won, with Dean shown in tears in Purgatory when he thought he'd lost Castiel. Their reunion was their most intimate yet, as the two didn't hold back on letting the other know that they were truly sorry and didn't want to fight anymore.
Eventually the two comforted each other and slept together, not realizing in a few short months, Meg would be carrying a Nephalem, their unborn child, Cassandra Master. Castiel learned of Meg's pregnancy and did everything in his power to hide her from angels or demons.
Season 15 - Love Confession
The season centers a lot around their relationship and it all comes to a culmination point when Cas makes the romantic declaration of love before sacrificing his life for Dean yet again.
Before dying, John tells Dean, if he can't save Sam, he'll have to kill him, should he become evil. The fourth season episode "On the Head of a Pin" reveals that the demon Alastair tortured John in Hell for over a century, with John refusing the demon's offer to stop if he himself would torture someone else.
The Alpha Vampire was the very first vampire and progenitor of all other vampires.
Emma Winchester is the daughter of Dean Winchester and Lydia.