The Undead army is utterly unlike a mortal force. It is wholly dependent upon its leader, the Necromancer, Liche or Vampire whose will binds it together. If the army leader is destroyed then the whole army is defeated - Skeletons will crumble to dust, spirits dissolve into the ether, and Ghouls scatter to their secret hiding places.

No March Moves

No undead troops other than the army's leader can ever make a march move. They move forward in a slow, shambling manner, driven by the will of their master. Undead troops lack the coordination to march in ranks in the normal manner, which makes them less mobile than living warriors. They might be imagined shambling forward in an unstoppable horde, a relentless mass that is inevitably slow to change its purpose.