Army List
See Graham about this Army

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In the Lord of the Rings books, the Army of the Dead (also called the Oathbreakers or Dead Men of Dunharrow) consists of ghosts who are bound to the Paths of the Dead by a curse from Isildur until they fulfill their oath. Their primary function in the book is psychological, using fear to defeat the Corsairs of Umbar at the Pellenor Fields and they do not physically fight or defeat the armies of Minas Morgul.
 

What they are

The Oathbreakers were Men of the White Mountains who swore an oath to Isildur to fight against Sauron but broke their promise. They were cursed by Isildur to remain as ghosts until their oath was fulfilled. Their spectral forms were bound to the Paths of the Dead, a cavernous pass through the mountains. 

How they are used

Aragorn, as the heir of Isildur, takes the Paths of the Dead and commands the Army of the Dead to follow him. Their main purpose in the book is to instill terror in their enemies, not to fight physically. They are not capable of sailing the ships they need to get to the main conflict, which is why they must only be used to defeat the Corsairs who were threatening to attack Minas Tirith. 
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The Army of the Dead defeats the Corsairs on the black ships with their psychological power alone. They then disappear after fulfilling their oath and Aragorn goes on to the Battle of the Pelennor Fields to fight alongside the living soldiers.​

 

The Army of the Dead wasn't a massive fighting force at the Battle of Pelennor Fields; it was a relatively small group of spectral warriors that, in the books, was used only to defeat the Corsair ships at the docks of Umbar. After the Corsairs' forces were scattered, the oath-bound dead were freed from their curse, leaving only the men and elves accompanying Aragorn to continue on to Minas Tirith. ​

Aragorn's arriving reinforcement was composed of the men with him: Legolas the Elf, Gimli the Dwarf, the sons of Elrond (Elladan and Elrohir), the Grey Company of Rangers and some reinforcements from southern Gondor.