In the (roughly) D&D based world of ..., The Abyss, which birthed magic, gives men the power to fulfill the deepest and darkest desires of their hearts, unleashing great suffering upon their brethren.
So, when mastered, necromancy can do some pretty wicked (cool) stuff. For necromancers, it allows them to raise an army out of nothing. While zombies have the same physical characteristics as ordinary humans, they can be created much faster, will always be loyal to their master, have no sense of fear, and can also be put into and woken up from hibernation at convenience.
So, yeah, that's how necromancy is. While it isn't inherently evil, is the hardest to master. This field typically attracts warlords like William Walker, who now have a way to raise hell in a certain region, then, when the king's army comes to give them a spank, he can easily bail, rinse and repeat. However, regular people can't handle this cycle, which would eventually lead to societal breakdown if left unimpeded.
These zombie armies are excellent at guerilla warfare and can swell up to ~10 000 in number, making them a rather significant force to deal with.
Ideally, a world with these zombie-making filibusters should look something like this. However, I'm struggling to create a "guarantee" that ensures a status quo where filibusters pose a threat to kingdoms, but can't easily take them over or cause lasting damage to settlements they take over.
What limitation should necromancy and/or the zombie army have to ensure that?
The world in which filibusters exist is the same, as far as the structure of society and technology is concerned, as medieval Europe.