The Witch-king or also known as the Black Captain was once a great king of men. He is considered Sauron's second in command and usually resided in Angmar. The Witch-King was present at the war of the last alliance but after Sauron was defeated he and the other 8 Nazgul went into hiding. The Witch-King was the one who pierced Frodo with the Morgul blade that almost turned Frodo into a wraith.
The Witch-King was in command at the battle of Minas Tirith. The Witch-King was wounded by Merry who stabbed him in the back of his leg and killed while distracted by Eowyn who swept her blade over his shoulder's (where the neck of a normal man would be) and being a woman fulfilled the prophecy that "no living man" could .
The power for Merry (a man) to wound the Witch-King came from his enchanted power of the blade which in The Fellowship of the Ring (book) was given to him by Tom Bombadil after Bombadil had rescued the four companions from the clutches of a Barrow-wight in the midst of what seemed to be a sacrificial ceremony within one of the Downs.
The Barrow-downs are located just west of the Old Forest which served as burial mounds for the men of the ancient lost realm of Arnor in the north, whose weapons served against the dark powers in the past and apparently still carried their magical potency to harm the servants of the Enemy.