The Witch-King of Angmar 

Some of the best justice in LotR comes with Éowyn's and Merry's slaying of the Witch-King of Angmar during the Battle of the Pelennor Fields outside of Minas Tirith.

The Witch-King of Angmar has many names--the Lord of the Nazgûl, the Captain of Despair, the Lord of Morgul. Even Éowyn gives him a name, Dwimmerlaik, which probably means something like 'zombie.'

When the Nazgûl returned to Middle-Earth in III 1300 in the northern lands of Angmar, they destroyed the lands surrounding Angmar, including the north-kingdom realms of the Éothéd and the lands of the Dúnedain.

After Angmar itself was destroyed as retribution, the Witch-King survived as leader of the nine Nazgûl. He emerged again during the War of the Ring as the captain of the great armies that lay siege on Gondor.

It was prophecied that "no living man" could kill the Witch-King, as he proclaimed aloud to young Dernhelm when the knight threatened attack. When Dernhelm removed his helmet to reveal his identity--not a he at all but Éowyn in disguise--the Witch-King was silenced in surprise.

Éowyn first beheaded the dragon-like flying beast that the Witch-King had ridden into battle, forcing the dark captain to face her on foot.

Merry, who was nearby and had seen Éowyn's disguise unveiled, rushed forward to help her. The Witch-King broke Éowyn's arm with his great mace, and he was about to kill her, but then Merry stabbed him in the back of his knee, enabling Éowyn to finally kill him.

Once this feat was accomplished, Merry was amazed to see the ghostly Witch-King vanish in a spirit-death, leaving behind only tattered rags and an empty crown.