The Undead Scourge is a well-balanced faction that can field enduring ground forces and powerful air units. Their spellcasters possess a variety of powerful magics, including the dreaded ability to raise fallen allies and foes alike into an army of walking dead. In this way, the Undead can field armies more numerous than any other race in Warcraft III.
Advantages

Summoned Buildings
The Undead do not build structures, but instead summon them into being. Once an Acolyte is told to build a building, it begins the summoning spell, and the building gradually appears. During this time, the Acolyte can return to its duties, since it does not need to stay with a building while it is being summoned. The Undead can also Unsummon their buildings and recoup a percentage of the gold and lumber cost originally used to summon the building.

Corpses
In addition to gold and lumber, the Undead Scourge harvest corpses as a tertiary resource. Corpses are not used for purchasing units or raising buildings, but instead are used to fuel some of the powers of the Undead. The Necromancer unit uses corpses to create Skeleton Warriors, while the Ghoul unit consumes corpses to rapidly restore health. Corpses disappear over time, but the Undead can store them in their Meat Wagon units, where they will not decay and can be unloaded for use when the need arises.

Defense
The Undead have very tough defenses thanks to the Halls of the Dead and Black Citadel which acts as a super tower. Their Spirit Towers are also very tough and can be repaired during battle.