I know very little for sure about wraiths. I have only seen them in the city of Boston, where I recently moved to go to college, though they may exist elsewhere. One in particular, in the Boston commons, seems to have gained somewhat of a reputation as a well known ghost, but it does not seem stronger for it.

Wraiths are baffling entities, because they seem to be existing as inactive spirits, but survive indefinitely. They also seem to be physically manifesting continuously. The area around a wraith will be several degrees colder than the surroundings, most noticeable directly next to the entity, and growing plants and flora seem to have difficulty growing in the cooled area.

Wraiths appear enormous, towering over all other spirits, and formless as smoke. They are unique in appearance as they appear black, rather than the pale monochrome I normally see ghosts as, and they are not transparent at all. They move in a liquid manner, flowing as if made up of a huge curtain or cloak of fabric, and have no discernable facial features or limbs.

Other spirits avoid wraiths, even predatory hunters.

I do not know if wraiths are harmful to living humans, though they should probably be avoided. They do not appear to follow humans as parasites do, or interact with spirits.

My only working theory for how they continually exist is that they somehow drain the surroundings, the world itself of residual energy. I cannot explain how.