The witch-king of Angmar

The Witch-king of Angmar was the leader of the Nazgûl or Ringwraiths, and Sauron's second-in-command in the Second and Third Ages. 
Once a king of men, he was corrupted by one of the nine Rings of Power given to the lords of men and became an undead wraith in the service of Sauron.

After the first defeat of Sauron in the War of the Last Alliance, the Witch-King fled to Angmar, a kingdom he ruled for over thousands of years until he returned to Mordor to lead Sauron's armies in the War of the Ring. He was killed in the Battle of the Pelennor Fields by Meriadoc Brandybuck and Éowyn niece of Theóden at the end of the War.