The Nazgûl (also known as Ringwraiths) are the chief servants of the Dark Lord Sauron (Faylynn). They once were nine great Kings of Men, but fell after they were all given Rings of Power. The Nine Rings consumed them, and they became servants of Sauron, his chief lieutenants, once he gained control through his newly-forged One Ring. After a long time, the effect of the Nine Rings left the kings spectral, invisible to all but those who could see into the wraith world (such as high elves or the bearer of the One Ring).
On the server, the Nazgûl fill more or less the same capacity. There will be Nine in the future: six of Mordor and three of Dol Guldur. There are currently four. Player okurskull serves as the Witch-King of Angmar, alongside canacx, who serves as the Despised, both for Mordor. Dol Guldur has two as well: Khamul_The_Black as Khamûl, the Black Easterling, and the__overlord, as the Gloom of Núrn.