Undead is one of the most feared armies in the Warhammer World. I'm not quite sure what areas of the world that they occupy are, so I can't tell yoiu that, but I do know that they are almost everywhere there are Necromancers, or people who can raise dead. The Undead army consists of Skeletons, Mummies, Liches, Vampires, Whights, and many other Undead sorts of things. They are not a powerful hand-to-hand combat race, instead they mostly rely on magic. Magic is their stronghold because thats how they get their armies, by raising the dead.
Here are some tips for playing the Undead army. First of all, never march move, this will cause your forces to shatter. Secondly, keep your General relatively protected, if he dies, then your whole army is destroyed. I would suggest surrounding him with troops, and keeping him away from combat and out of the range of archery until you know you are going to win. When you pick your spells, be sure to pick good ones such as Curse of the Years. That spell will tear up enemy units.
Here are some tips for fighting against an Undead army. First of all get some protection from magic. Since Necromancers are allowed to pick their spells, they will probably pick some Raise Dead spells, and Curse of the Years. If you have any Destroy Magic Scrolls on your wizard, I would suggest using it on this spell. What it does is kills models in a unit on a 5 or 6 on the first turn, and keeps getting better every turn. Secondly, go after their general. If you can kill their General, their whole army dies instantly, therefore you've won. Finally, their whole army causes fear so be prepared for that.