Sorin Markov, the ancient vampire Planeswalker, has returned to his home plane. But the heir to the House of Markov is not welcomed by his kin, nor does he expect to be. Those who know the secret of his past revile him. On Innistrad, Sorin Markov is a traitor.
The Markov family is one of the most powerful vampire houses on Innistrad, and Sorin was raised in a "life" of utmost privilege within the walls of Markov Manor in Stensia. Over many centuries he saw vampires becoming gradually bolder in their mistreatment—and sometimes outright slaughter—of the humans of Innistrad.
The vampires were growing stronger and stronger, feeding on the humans to such extremes that the continued existence of the human race was in question. Sorin Markov, already a powerful Planeswalker by then, had enough foresight to see how this would end. The humans would be hunted to extinction, which would cause the vampires themselves to die off. Sorin knew he must find a way to prevent this from happening.
Sorin disappeared from Innistrad for months, even years at a time. Other vampires assumed he was a hermit, an eccentric antisocial prince. But unknown to his kin, Sorin was traveling the Multiverse, searching for a solution to the grave situation on his home plane. After years of research, he returned with a revolutionary plan—one that would make him a pariah to the very vampires that he saved.
Sorin established a religion with real power to help the humans hold their own against the forces of darkness. He created a living covenant, an angel named Avacyn who could keep a balance between humanity and its predators. In actuality, Sorin's faith was a series of enchantments and spells designed to protect the humans of the plane. His work was successful. It gave the humans the edge they needed to come back and repopulate. And over centuries the religion developed and flourished, becoming more layered and complex than Sorin originally anticipated.
Sorin knew that he wouldn't stay on Innistrad permanently to maintain the enchantments that comprised the religion, so he set up a way to "refresh" the magic in his absence. He devised a system of wards and spells that, when cast repeatedly, added strength to the overall protective magic of the faith. In order to encourage the humans to say the wards and keep the enchantments strong, he made Avacyn the focal point of worship and incorporated beliefs about the moon and afterlife that were already prevalent in the world.
But now his angel has vanished and the dark threats of Innistrad close in on its people once more. Sorin has returned, intent on finding Avacyn and stopping the extinction of the human race.