Centuries ago, before the current history of Middle-earth took place, there lived nine great kings of men.
When the first Rings of Power were crafted, each of the kings received one. Soon, all of them began to obsess over the rings. Their obsessions became so far in excess that, one by one, each king began losing pieces of their identities. After spending so many days and nights being dazzled by the influence of their rings, these kings no longer knew who they were or what purpose they served. However, they did know one thing...they had become the loyal servants of the Dark Knight Sauron..they would give their very lives if it meant furthering Sauron's twisted purposes...they were the Nazgul--head soldiers for the vile Dark Lord!
The sole purpose of the Nazgul (or "Ringwraiths") was to find the One Ring and return it to its master. They were not going to spare anyone or anything in their pursuit to reclaim this ancient treasure.
The nine gave their undying allegiance to Sauron until his ring was finally destroyed in the flaming pits of Mount Doom (with Sauron being destroyed along with it). The Ringwraiths, neither living nor dead, were said to have simply vanished after the fall of Sauron, never to be seen or heard from again. However, they were destroyed by a fiery plume of magma while flying over Mount Doom to reclaim the Ring, once they knew where it was and who had put it on.
One Nazgul stood out from the rest. This one eventually became leader of the pack, and was known thereafter as the "Witch-king of Angmar," Sauron's most trusted advisor and commander. Although he was the most powerful of the nine Ringwraiths, he would often travel with his fellow Nazgul not in the dark, dingy
armor and helmet that made himstand out so, but as a rider in black like the rest. When the War of the Ring began to explode across Middle-earth, the Witch-king would often travel alone on his winged steed and signal the other 'Wraiths from their stronghold Minas Morgul to battle.
The Witch-king was defeated in battle by Eowyn during the Battle in Pelennor Fields.