Very little is known or talked about the origins of the king who took the ring of power from Sauron, this quote from Gandalf is the best description that we have:
The Witch King was once a man, that was corrupted by one of the nine Rings of power forged by and given to the lords of men by Sauron in the Second Age. He and eight other Men received incredible power from the rings, which they used to further their own ambitions, but eventually the Rings completely corrupted them and turned them into wraiths, undead creatures, slaves to the will of Sauron.
Their wraith like forms were the reason for their alternate names: Ringwraiths.
One thousand years into this age, Sauron took a new form as the necromancer, and founded the fortress of Dol Guldor in southern Mirkwood. This signaled the arrival of the Nine Nazgûl to Middle-earth.
The Lord of the Nazgûl reappeared a few centuries later in the north near the lost realm of Arnor. There he founded the kingdom of Angmar. It is after the formation of Angmar and several conflicts with the Dunedain (Gondorians) of the North that the Lord of the Nazgûl received the title of Witch-king, Lord of Angmar. He was responsible for the destruction of the few remaining cities in Arnor and became both a skilled strategist and a cunning tactician much like his master.
One city in all of Arnor resisted his grasp: Arthedain. This was because the King of this city, had earned the support of Lindon and Rivendell, and in particular the most notorious warrior of Lindon; The Balrog slaying Glorfindel.
He and Earnur met the Witch King on the field of battle, in the Battle of Fornost. The Witch King challenged Earnur to single combat. Earnur's horse fell and ran away out of fear. After much taunting the Witch King fled, Glorfindel prevented Earnur following and uttered a propheic warning:
"Do not pursue him! He will not return to these lands. Far off yet is his doom, and not by the hand of man shall he fall."
War Of The Ring
He met Glorfindel once again here, when he and 2 other Nazgul were pursuing the Hobbits as they made for Rivendell, Glorfindel appeared and drove them off allowing the Ring-bearer to escape his clutches, however not before wounding Frodo with his poisoned Morgul blade.
Feats include, capturing Minas Ithil and transforming it into his new lair: Minas Morgul.
He also planned the attack and capture of Osgiliath, the capital of Gondor:
He orchestrated the siege of Minas Tirith, which only failed due to the intervention of The Army of The Dead. He also met Gandalf in single combat here, however stalemated him before retreating.
It was here where the Witch King descended upon Pelennor fields and killed Théoden son of Thengel, King of Rohan. However, as he prepared to deliver the final blow to Théoden he was stopped by Éowyn and Merry. Éowyn slew his fell beast in two strikes. The Witch-king arose filled with malice, attacked, and though a powerful warrior, Éowyn was no match for the Witch-king's might. With a single blow of his mace (flail in the movie), he shattered her shield and broke her left arm.
The Witch-king slain by Éowyn and Merry.
As he towered over her, preparing to deliver the final blow, Merry snuck up behind him and plunged his sword into the back of Witch-king's knee. The sword, made in Westernesse centuries ago, broke the magic of Sauron that kept the Witch-king anchored to this world. While the Witch-King was distracted, Éowyn drove her sword where the head of the wraith would have been, slaying him.
The warning of Glorfindel to Eärnur so many centuries before had ironically come to pass. For not by the hands of a "man" had he fallen, but by those of a woman and a hobbit. With his death, the tide of the battle had changed and ultimately the outcome. No more than ten days later, Sauron himself was finally defeated when The One Ring was finally cast back into the fires of Mount Doom.
Powers and Ability's
As a Nazgûl, the Witch-King has a wide arsenal of weapons. First of all he has a sword and is extremely skilled with it. He also has a mace and is powerful enough to shatter a shield with it in a single blow. Appart from normal battle weapons, he also possesses a Morgul Blade. This dagger-like sword has the ability to turn anyone who has been stabbed by it into a wraith, but only after a few hours. The swords blade turns to dust after it has tasted flesh. The only known cure to it is elfish medicine.
The Witch-King is a ghost-like creature. This means he is nearly invulnerable, with the exeption of fire. This doesn't kill them but will surely scare him away. The Witch-King claimes that no man can kill him. This is very litteral since he was killed by a woman, Éowyn. It's not known if he could be killed by other women. He's also invisible for everyone who's not wearing The One Ring, the robes and armor he wears is still visible though.
One of his famous ability is his screech. Almost everyone who hears it becomes paralyzed with fear and mostly their only reaction is to cover their ears. Even hardend Gondorians, who fought bravely every day against the forces of Sauron, which includes the Witch-King as his high commander, couldn't stand. Only after about half a minute they were capable again of fighting. The more far away the Witch-King is and the more occupied the person is the less effective the scream is. For example: If the Witch-King is flying very far away, or the person is very busy with fighting an enemy, the scream has less impact.
The Witch-King was also a feared sorcerer who has powers over the physical world. He even broke Frodo's sword with a move of his hand and he weakened the gates of Minas Tirith, allowing Grond to break it. The Witch-King has more magic, but it's not discribed what this magic is. The only thing for sure is that it's powerful and among the ability's are some curses.
The Black Breath
Another ability the Witch-King has is called the Black Breath. Instead of being really the breath of the Witch-King, it's more like an aura he can produce at will to incapacitate foes. A mild form of Black Breath may cause unconsciousness accompanied by bad dreams and a state of agitation upon being woken. In more severe cases it can cause death, if not treated by the special herb Athelas. Symptomes are deep despair, extreme drowsiness, graduatly losing consciousness or immediately losing consciousness, hallucination-like nightmares, hypothermia and after several days death.
The Witch-King used two types of steeds:
The first one was a horse. It was a black horse. It is said that the Nazgûl stole their horses from the Rohirrim. This may imply that the Nazgûl, which include the Witch-King, had multiple horses, though this is not sure. The stolen horses were further bred inside Mordor to endure such a rider. The horse wasn't magical, but probably stronger and faster then our regular horses.
The second one was a terrible, flying, pterodactyl-like creature, often called a Fell-beast. Other names were Hell-Hawks or Nazgûl-Birds. The Nazgûl started using them when their horses washed away in the Fords of Bruinen. They had claws and a beak. They didn't had any feathers, they had a long neck and their body gave off a stench. The creature was likely bred by Sauron in order to supply the Nazgûl with steeds. The Witch-King's Fellbeast was slain by Éowyn in the Battle of the Pellenor Fields. The people of Minas Tirith later burned the body and on that place wasn't anything that wanted to grow there.