As Supernatural prepares to return for its 15th, and final, season, it's difficult to keep track of the countless number of monsters the Winchester brothers have fought over the years. Sam (Jared Padalecki) and Dean (Jensen Ackles) have taken on everything from simple ghosts to actual gods. So there's no harm in needing a refresher now and then. Among those that may have fallen through the cracks are the Alphas, who debuted in Season 6.

Children of Eve, the Alphas are the progenitors of their respective monster type: vampire, werewolf, shapeshifter, and so on. As their name implies, they are the strongest of their species, and are generally immune to weaknesses that affect their monster type.

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The Winchesters first encounter an Alpha in the episode "Two and a Half Men," when Sam investigates a case involving a series of missing babies whose parents are found dead. While at a crime scene, Sam discovers a left-behind baby and recruits Dean to help him. The brothers decide to take the infant to their grandfather, Samuel Campbell (Mitch Pileggi). As they debate what to do with the child, the Alpha Shapeshifter appears and takes the baby.

Two episodes later, in "Live Free or Twihard," Sam and Dean come face-to-face with another Alpha. While investigating a group of vampires that has taken up residence in a small town, Dean is bitten and turned into a vampire. To change back, Dean needs to acquire the blood of the bloodsucker that changed him, leading him to infiltrate the nest of the vampires in the town. While there, Dean runs into the Alpha Vampire.

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Later in Season 6, the Alpha Shapeshifter reappears, as a prisoner of Crowley, who is searching for Purgatory. Because of their connection to Eve, the Alphas are aware of the existence of Purgatory and the location of its gate. When the shapeshifter refuses to give up the location of the gateway, Crowley beheads him. When Castiel arrives, he frees the rest of Crowley's prisoners. At that point, it's mentioned several of the Alphas had been killed while subjected to Crowley's interrogation. That was the last viewers would hear about the Alphas quite some time.

The children of Eve didn't appear again until Season 12, in "The Raid," when the British Men of Letters began Project V, an operation aimed at exterminating all American vampires. Their crusade killed 230 of the 241 vampires in the Midwest, and caught the attention of the Alpha Vampire (Rick Worthy). The vampires retaliate and attack the British Men of Letters compound, killing most of them. Sam, who was at the stronghold at the time of the assault, gathers the survivors and begins to prepare for a final battle. During the confrontation, Sam shoots the Alpha Vampire through the head with the supernatural revolver, the Colt, and killed him.

Returning for its final season Thursday, Oct. 10, on The CW, Supernatural stars Jensen Ackles, Jared Padalecki, Misha Collins and Alexander Calvert.

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