The Witch-king

The Witch-king was once a great king of Men before being gifted one Ring of Power. Falling to its corrupting influence, he became the Lord of the Nazgûl and the deadliest servant of the Dark Lord Sauron himself.

Attempt on Frodo's life
The Witch-king was one of the five Ringwraiths that ambushed Frodo Baggins and his companions at Weathertop.

He wielded a long sword and a Morgul-blade that he used to wound the hobbit in an attempt to retrieve the One Ring for Sauron. He was forced to retreat when Aragorn defended the group with a torch.

To Mordor we'll take you
The Nine Nazgûl tracked their quarry to a forest on the road to Rivendell, separating Frodo from the rest of them. He managed to evade them by making it through the river. The Witch-king tried to convince the hobbit to give them the Ring willingly, but when denied, the wraiths charged at him, by attempting to cross over.

However, Gandalf the Grey and Elrond sent a magical flood that stopped the Black Riders from reaching their prey.

Trivia

 * In Ralph Bakshi's The Lord of the Rings, he is only identified in the credits and by the role he plays, as he is not named aloud in the animated film, similar to how he is revealed to be a much more sinister character in the third book written by J.R.R. Tolkien.
 * He wears the same armor and cloak as the rest of the Nazgûl, but has a unique helmet.