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. The rest of the time they must remain in the Moon Lay. Nightwraiths are beings who remained too long in the Moon Lay and became trapped there. Nightwraighs cannot interact with anything physical in the material realm, except items that have been specifically enchanted to enable them to do so, such as weapons.
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.
Two such beings were used by Atury to steal the Inar Stone, which was taken through the gateway so he could possibly study it.