Supernatural's Sam and Dean Winchester have made a career out of "saving people, hunting things." But monster hunting is a dangerous vocation. Sam and Dean risk life and limb coming into contact with things that go bump in the night.
Even worse, on several occasions they've transformed into monsters themselves, some of the most dangerous transformations of which we're re-living in roughly chronological order below.
SAM IS "SPECIAL"
When Sam was six-months-old, the yellow-eyed demon, Azazel entered his room and fed him demon blood. The demon blood Sam consumed gave him powers that began manifesting themselves in young adulthood. Sam learned that he was one of a group of "special" children who all exhibited powers after being given demon blood as babies. Azazel gathered the group and made them compete in a Hunger Games-style competition. Whoever survived was supposed to release and lead Azazel's demon army. Sam was killed during the competition, but was resurrected when Dean made a deal with a crossroads demon.
Azazel died shortly afterwards and Sam’'s powers seemed to disappear. However, once the brothers met the demon Ruby, she convinced Sam to train with her. This is how he learned the depths of what he could do. Sam was telekinetic and could exorcise and even hurt and kill demons using just his mind. And he was especially powerful when he drank demon blood.
However, he soon became addicted to the substance and was colder and more calculated under its influence. He lies, takes questionable actions and is completely enthralled by Ruby. It's not until Ruby reveals her true endgame that he realizes his mistake and begins to work to redeem himself.
SAM IS LUCIFER'S "MEAT SUIT"
At the end of Supernatural's fourth season, Lucifer is released from Hell and early in Season 5, he reveals that Sam is his true vessel. However, angels need permission to possess someone, and Sam refuses to consent. That is, until he hatches a plan to send Lucifer back to Hell. He proposes that if he says yes to Lucifer, when the time is right, he can regain control of his body just long enough to drop Lucifer and himself into Hell by using the rings of the Four Horsemen of the Apocalypse.
Once possessed by Lucifer, Sam seems completely taken over. However, when Lucifer beats Dean to a pulp, Dean's words of solidarity and a glimpse of a toy soldier that got stuck in the ashtray of the Impala somehow break through to Sam. This allows him to regain control of his body just long enough to enable him to open Lucifer's Cage in Hell and jump in.
Shortly after Sam jumps into Lucifer's cage, he's resurrected. However, when Sam comes back, he no longer has his soul. Soulless Sam knows the difference between right and wrong, he just doesn't care. In fact, he's completely emotionless and the allegiances he once had to those in his inner circle, including his brother, are no longer important to him. Sam argues being soulless makes him a better hunter, however, it also makes him exceedingly dangerous. At one point he even tries to kill Bobby.
Dean turns to Death for help and Death restores Sam's soul. Still, the horrible things Sam experienced in the cage with Lucifer and Michael continue to be problematic for Sam in future seasons.
During the time that Sam's without his soul, he and Dean go to investigate a possible vampire attack. Dean is accosted by one of the vampires, and instead of helping him, Sam looks on as Dean is turned. Dean is horrified by his situation and overwhelmed by his heightened senses. He asks Sam to kill him but Sam refuses.
The brother's newly resurrected grandfather, Samuel Campbell, informs them that there is a cure for vampirism as long as the newly turned vampire hasn’t fed on human blood, which Dean has resisted with some difficulty. The cure also requires the blood of the vampire who turned him, so Dean goes to the vampire nest and attains his prize. With the cure, Dean makes the painful transition back to humanity.
SAM IS HEALED BY GADREEL
When Sam stops pursuing the trials to close the Gates of Hell, it leaves him comatose. The doctors inform Dean there's nothing they can do, so Dean prays to the angels for help. An angel who calls himself Ezekiel answers Dean's prayer but claims Sam is so deeply damaged that he would have to possess him in order to heal him from the inside. Dean and Ezekiel agree to this arrangement but keep it from Sam.
At first, the arrangement seems to be working. Soon though, Dean learns that the angel is actually Gadreel, who has been imprisoned since the dawn of time for letting Lucifer into the Garden of Eden. Gadreel is trying to atone for his sins but ends up following Metatron, the scribe of God, in his bid to become the next God. In the course of following Metatron's orders, Gadreel kills the prophet, Kevin and takes control of Sam. In the end, Castiel and the demon Crowley help Sam expel Gadreel.
DEAN BEARS THE MARK OF CAIN
Crowley and Dean work together to take out Crowley's competition for Ruler of Hell: Abaddon, a Knight of Hell. However, only the First Blade can kill her and the Blade only works for the individual who bears the Mark of Cain. Dean agrees to take the Mark even though Cain warns him of its insidious effect.
Soon Cain's warnings come true. The Mark amplifies Dean's aggression and gives him an insatiable urge to kill.
When Dean takes on Metatron while bearing the Mark of Cain, Metatron kills him. However, the power of the Mark won't let Dean die, at least not entirely. Instead, Dean wakes up as a demon.
In demonic form, Dean doesn't have a care in the world and feels no remorse. He drinks, fights and goes home with any woman he wants. He no longer worries about Sam, hunting monsters, or anything else. Sam eventually captures and cures demon Dean.
DEAN IS POSSESSED BY APOCALYPSE WORLD'S MICHAEL
Dean refused to be the vessel for the Archangel Michael in earlier seasons of Supernatural. However, after Lucifer steals his Nephilim son's (Jack) grace and whisks him and Sam away, Dean makes a deal with the Michael from the alternate universe, Apocalypse World. Dean agrees to let Michael possess him as long as Dean remains in control while they fight Lucifer. Then Michael will leave.
Although they successfully take on Lucifer, when the fight is over, Michael reneges on their deal and takes over Dean's body. In the time that follows, Michael works to destroy the world. Dean manages to lock Michael away in his mind for a time, but once Michael escapes, he's killed by Jack.