[Fenrir]

Here's a quote from the Tolkien Bestiary. It should be quite reliable.

"Barrow-Wights: West of the Brandywine River beyond the Old Forest were the Barrow-Downs, the most ancient burial ground of Men in Middle-earth. There were no trees or water there, but only grass and turf covering dome-shaped hills that were crowned with monoliths and great rings of bone-white stone. These hills were the burial mounds that were made in the First Age of Sun for the Kings of Men. For many ages the Barrow-downs were sacred and revered, until out of the Witch-kingdom of Angmar many terrible and tortured spirits fled across Middle-earth, desperately searching to hide from the ravening light of the Sun. Demons whose bodies had been destroyed looked for other bodies in which their evil spirits could dwell. And so it was that the Barrow-downs became a haunted and dread place. The demons became the Barrow-wights, the Undead, who animated the bones and jewelled armour of the ancient Kings of Men who had lived in this land in the First Age of Sun.

The Barrow-wights were of a substance of darkness that could enter the eye, heart and mind, or crush the will. They were form-shifters and could move from shape to shape and animate whatever life-form they wished. Most often a Barrow-wight came on the unwary traveller in the guise of a dark phantom whose eyes were luminous and cold. The voice of the figure was at once horrible and hypnotic; its skeletal hand had a touch like ice and a grip like the iron jaws of a trap. Once under the spell of the Undead the victim had no will of his own. In this way the Barrow-wight drew the living into the treasure tombs on the downs. A dismal choir of tortured souls could be heard inside the Barrow as, in the green half-light, the Barrow-wight laid his victim on a stone altar and bound him with chains of gold. He draped him in the pale cloth and precious jewellery of the ancient dead, and then ended his life with a sacrificial sword.

In the darkness these were powerful spirits and they could be held at bay only with the spell of strong incantations. They could be destroyed only by exposure to light, and it was light that they hated and feared most. The Barrow-wights were lost and tortured spirits and their last chance to remain on Middle-earth depended on the dark security of the burial vaults. Once a stone chamber was broken open, light would pour in on the Barrow-wights and they would fade like mist before the sun and be gone for ever.

Whew...