The Witch-king of Angmar is the leader of the Nine, once a great king of Men, he fell into Sauron's influence after getting one of the Rings of Power, becoming the leader of the Nazgûl. He is Sauron's second-in-command. He is the secondary antagonist of Middle-earth: Shadow of War, and has been seen mounted upon a drake.
First Among the Nine
"Hinder me? Thou fool. No living man may hinder me!" -- The Witch-king of Angmar
Sauron's chief lieutenant, the Witch-king of Angmar is the most powerful of the nine Ring-wraiths. For more than four thousand years, he had served as Sauron's most trusted servant, bound to him by the Ring of Power he accepted during the Second Age.
The Witch-king has a particular hatred for Gondor, having warred against it centuries ago. After seizing he Northern Kingdom of Angmar, the Witch-kings armies moved across Middle-earth's northern regions. An alliance of Gondorian soldiers and Elves routed them, though the Witch-king himself escaped. The Gondorian prince, Eärnur, wanted to chase the Witch-king, but the Elf Glorfindel bade him to stay. Since then, he aspired to become the ruler of Minas Ithil.
Chastened by the defeat, the Witch-king employed a different strategy, one that exploited Gondorian pride. He re-emerged years later when Eärnur had become Gondor's king and challenged him to single combat. Eärnur accepted and rode eastward alone, never to be seen again. The Witch-king has been unchallenged in combat -- and Gondor has been without a king -- ever since.
If Celebrimbor is said to be the nemesis of Sauron, then Talion is certainly the nemesis of the Witch-king. It was not enough to see Talion destroyed like other men and so the Witch-king pursued him with an obsession bordering on mania. He would not kill Talion, but make him one of the Nine.