Monsters: There are many monsters in the world, mentioned in numerous mythologies from different countries. There are old standards like ghosts, vampires, and werewolves, plus some you’ve probably never heard of before like rugurus. Here are a few that have caused the most trouble for Sam and Dean.
Eve (Julia Maxwell): Known as the “Mother of All,” she was the creator of all supernatural beings, which go to Purgatory after their deaths. She created the Alphas, the first born children of each monster species. She was resurrected from Purgatory to stop Crowley from his plan of stealing the souls of her children. Only phoenix ashes can destroy her, which Sam and Dean managed to track down with a little help from time travel and Samuel Colt, and they succeeded in killing her with it.
Leviathans: One of God’s first creations, they turned out to be a little too on the evil and destructive side so God locked them all in Purgatory. It was hinted that Eve may have been one of them. They are very difficult to kill and are only susceptible to Borax, which burns them. Even cutting their heads off only slows them down. They were brought back to Earth when Castiel took the souls of Purgatory into his body to gain power and the Leviathans hitched a ride. They stayed inside Castiel even after he returned the souls, but his body couldn’t maintain them for long so they left, seeping into the water supply as black goo and taking on humans as hosts. Their leader, who they have followed since the beginning of their existence and without whom they would just be regular monsters, took on the form of Dick Roman (James Patrick Stuart), a wealthy businessman. The Leviathans’ ultimate plan is to turn humanity into a docile race for them to feast upon. Other than one Leviathan eating another, or eating itself, the only known way to kill them is with “a bone of a righteous mortal washed in the blood of the three fallen.” The three fallen include a fallen angel (Castiel), the ruler of fallen humanity (Crowley), and a father of fallen beasts (an Alpha). Sam and Dean hunted down all the necessary ingredients and went after Dick, but killing him dragged Castiel and Dean into Purgatory.
The Horsemen: The Four Horsemen of the apocalypse: War, Famine, Pestilence, and Death. These guys wreaked havoc in their own special ways leading up to the apocalypse. They also wore special rings, like they were in an apocalypse club together except instead of decoding things the rings were the key to Lucifer’s cage. Sam and Dean tried to collect them all (à la Pokémon) so that they could lock Lucifer back up. They cut the rings off War, Famine, and Pestilence with the help of a near-human Castiel. However, Death, who Lucifer freed from a magical coffin deep within the earth, willingly gave his ring to Dean. He did this because Lucifer had him bound with a spell to do Lucifer’s bidding, and Death hated being controlled by what he considered to be a child having a tantrum.