The Witch King is the foremost and most dreadful of all of Sauron's servants. He is also the captain of Sauron's armies. He is likely originated from the Black Numenoreans who settled in Middle-earth before the fall of Numenor. In the Second Age, he was one of the powerful kings who received the 9 ring recovered from the Elves. In time the ring corrupted and changed them, turning them into wraiths, spirit slaves of Sauron.