As is typical, the dead rise up from their graves and shamble around, seeking to slay the living. Civilization collapses and chaos reigns. After a few months of this, the survivors see three great glowing presences in the sky, which identify themselves as Michael, Gabriel, and Raphael, the gods of humanity. Long ago, they used to visit the Earth regularly and do battle with the dark god Lucifer, but that ended when all swore to a pact to leave humanity to its own devices. After centuries, Lucifer has found a loophole, and has drafted the dead into his dark army, allowing him to make war without setting foot on Earth. After some thinking, they have decided that they can imbue some of the living with their power without breaking the terms of the contract.
Across the world, hundreds of humans develop awesome miraculous powers. The Seraphim, chosen of Raphael, are wise and crafty, having access to boundless lore, and help humanity to rebuild what they have lost. The Cherubim, chosen of Michael, are leaders and commanders, able to assemble the shattered mass of humanity back into a mighty force to be reckoned with. The Ofanim, chosen of Gabriel, are mighty warriors, capable of striking faster than can be seen, and protectors of their fellow men.
The zombies are decimated and driven back. Of course, Lucifer starts unleashing increasingly virulent plagues and horrible beasts upon the Earth. You go from slow, walking dead zombies to fast, highly infectious zombies to werewolves to who knows what.