Khamûl was one of the nine Ringwraiths, second only to the Witch-king himself. He was called Sauron's lieutenant, "Shadow of the East", and "the Black Easterling". He was a king of Easterlings before being given one of the Nine Rings. He was the only Ringwraith whose name was given by Tolkien.
Khamûl commanded the fortress of Dol Guldur after Sauron's return to Mordor. It was he who asked Hamfast Gamgee for "Baggins" the day after Frodo Baggins had left Hobbiton and pursued the Hobbits to Bucklebury Ferry in the Shire some days later.