I have a little bit of the storyline in mind, but not a lot yet (I keep getting distracted). I'm not entirely sure whether I want to set it up so that people are fighting to survive the undead, playing as the undead, or both. The introduction/beginning story isn't what's bothering me so much as how to set it up. As the undead are organized, and not just a flailing mob of animated bodies, this could be difficult. I'm also unsure as to whether I want the undead to each have their own mind or be completely controlled by their necromancer/lich controller.
(A lich is what happens when a necromancer extends his life far beyond when the body would die. The body begins decomposing while it's still in use, and after a while there's nothing left but the skeletal frame and the necromancer's power. Becoming like this increases the power of the necromancer because now he has a much smaller bridge to gap between the living and the dead, and it becomes easier for him to control and create his monstrosities, as he himself is already essentially dead. The only other way for a lich to be created is by having a necromancer bring another necromancer back from the dead. Usually this would result in either the necromancer submitting to the lich, forcing the lich to submit to his will, or one of them killing each other.)