Zombie MMORPG in the Works
Warcry has a very short blurb about a just announced Horror-genre Massively Multiplayer game. Titled Exanimus, the game looks to be set in a post-apocalyptic zombie setting. From the piece: "While most of the world is dead, there are small pockets of survivors that exist in barricaded cities across the globe. Players of Exanimus will have the option of playing as a living survivor or as one of the dead roaming the earth. Exanimus will allow players to experience a MMO based in a horror genre by creating a dark and visually "intense" environment. Exanimus is expected to be rated "Mature" and not intended for anyone under the age of 18."
Simple solution: there's a limited number of lives in the game. In order to have a life, you have to take it from someone who has it. That is, kill a living player and you become one, and he becomes a zombie. Now you just need to have the server control the ratio between living and dead by automatically making some people living and some zombies at the start...
A zombie cannot be permanently killed, but will always raise back up again after a short while. However, a zombie can't earn exp, and will always be "level 1" - if you had levels as human, you lose them when you become a zombie, and regain them when you regain your humanity. Same for items. That way, all the zombies will desperately want to become humans, and because they are so much weaker, pretty much have to attack en masse. Since zombies can't level, the various grinders are sure to fight tooth and nail to become human again.
As a practical matter, you don't immediately become a human when you kill one, it takes five minutes (with a running countdown). That way you have time to escape from the fight - otherwise you'd just be killed as soon as you turned human in the middle of the battle.
Logged-of humans fall unconscious. An unconscious human can obviously be easily killed by any zombie, so better find a safe location. No place is inherently safe (zombies can attack you everywhere), but of course the players can establish player-guarded resting areas in the ruins of abandoned cities (in fact I'd put the entire game into such a city), and of course zombies would consider those their main targets. People will still have to leave them to get exp, to scavenge for resources, and so on.
Oh, and the zombies can see in the dark, while the humans can't. To avoid cheating, a zombie in suitably deep shadow is invisible to the client as well - that is, the server won't send the geometry at all.
Of course all this requires a game engine that can take a lot of people in the same area at the same time. It would also require pretty low lag, since zombies hiding in the dark leads to surprise attacks. Alternatively, you could perhaps determine a "guard location", so that your character shoots as soon as a zombie pops to the guarded area - this would make the game more strategic than reflex test.
Anyway, that's how I'd do it.
Forget magic. Any technology distinguishable from divine power is insufficiently advanced.