At the time, only about 24% of the world believed in the supernatural. The rest of it, well they just didn't believe. They saw believers as insane, closed-minded, fear mongers. Little did the non believers know, that these "fear mongers" were correct to believe in the supernatural. One group of people who believed were the X-Men.
Being mutants with extraordinary abilities, they figured it was plausible that there are forces out there at work, that we can and can not see. A specific mutant that knew for a fact that a spirit world did in fact exist, was Professor Xavier; he could sense spirits both kindred, and vengeful. This force no longer needed to be sensed in order to be discovered; at that moment, it decided to make itself indubitably visible.
Lucifer had risen from his cage in hell, to take command of The Four Horseman of the Apocalypse, so the could carry out his genocide against all of humanity. The skies filled with clouds darker than the night itself. The X-Men at this point in time, only consisted of Professor Xavier, Wolverine, Colossus, Cyclops, and Jean Grey. The five were discussing the presence of a dark force that Xavier had sensed to come. Four of the X-Men look intently out of the window, as Wolverine went straight for the phone to contact the only two people he knows can help save the world from something like this; Sam and Dean.
Wolverine first met the brothers when he, Colossus, and Blade were attacked by the vampires. The battle resulted in them overcoming the creatures, and the death of Dracula. Dracula was decapitated by Wolverine, right before he was about to sink his teeth into an unconscious Sam. For saving his brothers life, Dean was extremely grateful and told the heroes that if there was anything that they ever needed, he and his brother would be there. It was time to repay the favor.
The Four Horsemen immediately took over the streets on their steeds, spreading pestilence, famine, war, and death. Sam and Dean arrive at Xavier's mansion to discuss a plan, before it's too late. Sam says the horsemen's power is in their rings, which all act as a key to Lucifer's cage in hell. He suggests that Xavier contacts Lucifer to gain his attention, while the four members of the X-Men go after the Horsemen, to steal the rings right off of their fingers. Then once they acquire the rings, they would be given to Dean to stand by as Sam would allow himself to be possessed by Lucifer, in order to overpower the horsemen, throwing them in the cage of Lucifer. Then, Dean could lock the cage with the four rings, and recite an incantation to release Sam from Lucifer, leaving only the evil spirits to be damned to hell for eternity.
The plan went into action. Jean attempted to use her telepathy on War, but was overcome with horrific images, too horrific to stand. Jean had an aneurism and died, before being trampled by the steed of War. Cyclops beyond devastated, rushed over to Jean's lifeless body, not paying attention to his surroundings. Deaf to the warning call of Colossus, Cyclops was decapitated by the scythe of Death. The metal clad mutant was filled with rage, and grabbed ahold of Death, causing his metal covered body to turn back into flesh, leaving him vulnerable to War's sword, which pierced through his back, and out of his chest. The third mutant had fallen.
Wolverine couldn't control himself, and gave into his animal instincts. Pestilence, War, Famine, and Death realign, waiting for Wolverine to walk into his death, as his friends had just did. Wolverine jumped at the Horsemen, and in one swift and bloody motion, Wolverine sliced off the hands of the horsemen that were bearing the rings. As Xavier was continuing to get the attention of Lucifer, Sam put himself out into the open, to be possessed by the father of destruction. Dean was on standby to collect the rings. Wolverine grabbed ahold of the hands, and quickly threw them one by one, into Dean's direction. As Wolverine turned around, Pestilence, instantly starved his body, causing his muscle to wither away; Famine brought disease to the Wolverine, causing his flesh to peel away, until the only thing left was an adamantium skeleton. War grabbed ahold of the skeleton, and destroyed what was left of it. The fourth mutant had fallen.
Dean gathered the rings from the unattached hands, as Xavier's powerful mind could no longer handle having contact with the father of destruction, and fell unconscious. Seeing Xavier as the only threat left, Death approaches Xaviers unconscious body, taking his life with a simple touch. The last mutant was gone. Simultaneously, Sam is being possessed by Lucifer. Once possessed, Sam knocks the horsemen unconscious one by one, unfazed by their defenses. He takes them to hell, and throws their limp bodies into the cage, and walks in, himself. Once he is in, Dean began to incite the the incantation. But before he could finish, he was overcome by the sickness that Pestilence had released into the atmosphere before being drub into the cage. Dean fell to his death, and Sam was to be locked in hell for eternity. Even though Sam was possessed by Lucifer, he had enough power to stop Lucifer from ever taking command of the Horsemen again, as well as stop Lucifer from ever leaving the cage, to threaten humanity again. After the events, and the sacrifices of our heroes, the world was finally able to live in peace, free from the clutches of the forces of evil.