A Wraith is a being that has shed (or been robbed of) his physical form by dying through the use of dark sorcery or died in darkness. The most infamous wraiths in Middle-earth were the Nazgûl, or "Ringwraiths", who were the nine kings of men given the Nine Rings by Sauron.
There are many restless spirits within Middle-earth who have not been able to find peace in death. Wights, wraiths, and ghosts of terror, Sauron the Necromancer calls to them all within the borders of Mordor.
Should he rebuild his power and be able to command this lost army of the dead he will be unstoppable.
Talion and CelebrimborEdit
The elf-lord, Celebrimbor was reduced to being a wraith by being tainted by the darkness of Sauron. Being an elf, he existed naturally in both the real world and the wraith-world simultaneously. After dying, he came to inhabit the wraith-world, yet was still bound within the walls of Arda.
Talion was executed by the Black Hand in a ritual of dark sorcery, which had the side-effect of sending him to the wraith-world. There, he met Celebrimbor, and the elf-wraith used his immense power to fashion a new body for Talion so that he could once again influence the physical world. By being bonded with Celebrimbor, Talion has the ability to see into the wraith-world, and through him, Celebrimbor can temporarily enter the physical world to allow Talion to perform superhuman feats of speed, strength, and agility.