The Undead are a faction (referred to as race) in AoF. The main concept of the Undead is quantity over quality. What separates the Undead from the other races is the ability to re-animate corpses using a wide range of spellcasters.
The Undead's primary ability is to re-animate corpses using units such as the Lich. In a battle with lots of casualties, an army can be raised in an instant and give opposing players trouble. Infantry can also consume a corpse to regain health.
Undead also specialise in tackling ranged units. Most key undead units (Ghouls and Skeletons in particular) have very high pierce armour, requiring other methods to take them down. If backed up by a Lich, counterattacking would be unwise with summon dead.
Corpses are a minor resource for Undead. It is best to trample any corpses that you may find when playing against Undead. That way, there won't be an army raised by a single Lich in the following turns.
Undead are incredibly vulnerable to a wide range of debuffs. Area damage catapults can completely mitigate the dangers of an undead army.
Quantity over quality play is incredibly vulnerable to area damage effects such as trample. Orc Shaman, Catapults, Uruk Axeman and Fireball are all deadly to an army primarily made up of skeletons.
Undead late game consists of cavalry (Spectral Riders and Death Knights). Order infantry production facilities to producing anti-cavalry during late game.