I thought of something else that might strike your curiosity (interest).
What if a cursed land was created when the map is spawned, like in a desert for example. It doesn't have to be very large. Nothing at first betrays its haunted nature.
Each haunted square has a small chance of generating undead as long as a non-barb unit remains on anyone of them. The longer intruding unit(s) remain in the cursed land, the more powerful the various spawned undead and the greater the frequency of their appearance. Sooner or later, the intruders will get killed. Leave the land untouched for several turns, and the counter "resets" itself back to the "weaker" skeletons.
Somewhere in the cursed land lies an ancient city ruin. It is invisible until the intruding unit enters its square. If the intruder explores the ruin, the entity controlling these undead appears in an adjacent square, along with a "circle of carcer" to emprison it and the intruder (four adjacent squares?). If it is a stack of intruders, only the most powerful unit remains while the others are kicked out of the circle.
If the intruder defeats the entity, the other undead are removed (they find resting peace in the world beyond), and the winner gets some treasure or a special power. A "Good" intruder might lower the AC. An "Evil" intruder might instead gain control of the entity in some way.
To prevent all the undead from overrunning the neighboring lands, they should have a duration of a few turns after they leave the cursed lands.