There are many different forms of undead that haunt Middle-earth, here are details of a few of the haunted creatures one may encounter in their travels through the darker parts of the world.
In Eriador, between the Old Forest and Bree, lie the Tyrn Gorthad, or the Barrow-downs as they are now known. A region of hills and downs, it contains many burial- mounds, some the tombs of ancient kings of the forefathers of the Edain, and others of the Dunedain who later ruled this land. In the middle Third Age, during the 1600s, when the plague came to the North and devastated the people living there, the Witch·king of Angmar sent evil spirits, wights, to dwell within the mounds and make the Barrow-downs a place of horror. Since that day, few folk venture into this dread region. Barrow-wights are horrible, undead creatures. Although they have bodily form, they are evil spirits who wish to slay the living. Powerful with in their barrows or at night, they cannot stand the touch of sunlight. They prefer to attack from surprise, using their spells and powers to confuse their prey, bind it with spells, and then take it into the barrow to slay it and consume its spirit.
Corpse Candles can create the illusion of being a whole, healthy creature by cloaking the hideous remains of its once-living body. They lurk in water and exudes a beckoning light that magically attracts victims. Corpse Candles and Corpse Lanterns are lesser and greater manifestations of the same basic sort. They are remnants of those who, already lying unquiet, were flooded in their graves.
The Nazgul, Sauron's most powerful and favoured servants, are men to whom he gave the Nine Rings long ago, Three of them, including their dread captain, were great lords among the Numenoreans whom Sauron corrupted to his service. The others were powerful kings and chieftains of the Men in Middle-earth who worshipped him or were dominated by him, Each of these Men were powerful warriors and sorcerers as well, in time and they became more powerful and wealthy still with their Rings. But the Rings lengthened their lives over centuries, eventually causing
them to fade and become Ringwraiths, lesser shadows beneath Sauron's great shadow.
Mewlips are an evil, semi-legendary race of exceedingly rare, cannibalistic spirits. These shadowy spirits exist to drink blood, which they need almost as much as other undead need life-energy. Denizens of graveyards, ruins, and other pleasant sites, they are often found surprisingly near inhabited areas, especially if they have not found much sustenance farther afield lately. Mewlips are rare even for Undead. Fond of the most noisome and darkest swamps and marshes, they have a silent tread and strangling hands, although they typically use jagged weapons of rusty steel or stone. They are deceptively human in appearance: however, their backs are horribly hunched and their skin shines with a clammy, greenish-brown pallor. Even their ragged clothing is moist and foul. Mewlips feed on most anything and they covet shiny objects, especially things of gold. It is said that such items remind them of uncursed life, of the nature of beings not in the thrall of Darkness.