The Witch-king was the Lord of the Ringwraths. He
was also known as the Black Númenórean in the books, or
Murezor in the card game, sometimes called
he was lord of that place. Some other titles used in the books
were Wraith-king, Wraith-lord, Witch-lord of Angmar, King of Angmar,
Morgul-king, Morgul-lord, Black Captain, Captain of Despair, the High
Nazgûl, Lord of the Nazgûl, and lord of the Nine Riders.
While still human, he lived in the Second Age as a sorceror King. Once changed by his ring into a wraith, he operated under Sauron, commanding his forces.
When Sauron was defeated, the Black Númenórean went into shadowy limbo until Sauron recalled him a thousand years later. Thus he rose again in 1300 of the Third Age as the Witch-king of Angmar, constantly warring upon Arnor for about seven centuries until 1974, the time of the destruction of Arthedain. The next year, his kingdom of Angmar was destroyed in the Battle of Fornost. This was not a great setback to his plans, as he was through with the important business there of destroying the Dúnedain's North-Kingdom.
Then he turned his attention in the year 2000 to the South-kingdom of Gondor, taking Minas Ithil and renaming it Minas Morgul, using it for his new base. For the next thousand years he harried the realm of Gondor.
In 3018, the most important work of all took his attention. The One Ring of Sauron had re-appeared, the key to victory for his side. He led the other Riders to the Shire. At Weathertop, he wounded Frodo the Ringbearer, and chased him as far as the Fords of Rivendell where the elves destroyed the Riders's links to the mortal world temporarily by using the river.
They were remounted on flying beasts of horror, no longer requiring secrecy. The Witch-king led the vast forces collected at Minas Morgul along with the Haradrim allies to attack the South-Kingdom. He slew King Théoden of Rohan, chief ally of Gondor, just outside Gondor during the Battle of the Pelennor Fields, by using the fear of his presence to terrify Snowmane, Théoden's horse, so that it reared and fell on the king.
Glorfindel's prophecy stated that no man could destroy the Witch-king, as mentioned by Gandalf to Denethor. On the Pelennor Fields, the Lord of the Ringwraiths finally met his end at the sword of the Shieldmaiden of the Rohirrim, Éowyn, niece of Théoden, and the blade of Meriadoc Brandybuck, Hobbit of the Fellowship of the Ring, neither of whom were of the race of Men.
Reference: Lord of the
Rings , Silmarillion