Making some kind of core to the undead empire could be a useful addition. Currently the issue is that the undead are incredibly unbalanced, due to their nature. At first they're incredibly weak, because there aren't enough bodies, then they reach a point where they can make headway and suddenly there are unlimited dead bodies to form new armies, and you win almost instantly in an exponential zombie cascade.
Having a core would give something for the lightbringers to attack, like you said, and give a reason to slow down the undead spawning, perhaps tying the max undead to the size of your empire's construction. I'll have to play around with it to see.