A nightwraith is a spectral being that can only access the Material Lay by moonlight, and yet even while there they are not wholly physical.
History
In the time before the Great Purge, some elves were capable of traversing the Moon Lay. These were known as Moontreaders. They fled to that realm to avoid being killed by the Tal Calar, and because they stayed too long they became nightwraiths.