Minas Morgul

Minas Morgul was a City/Fortress of Mordor, originally called Minas Ithil. As the easternmost fortification in the kingdom of Gondor, and a sister city to Minas Anor, Minas Ithil safeguarded the eastern borders of the Kingdom of Gondor and the capital Osgiliath from the forces of Mordor during the early part of the Third Age.

Minas Ithil

Minas Morgul was located in an upland valley at the feet of the Mountains of Shadow. It overlooked the region of Ithilien and controlled the only easily accessible pass through the mountains that led into Mordor, the pass of Cirith Ungol.

Sauron sent a huge army to Minas Ithil and after about a year of siege, it was captured and the White Tree was burned (Isildur recovered a sapling and planted it in Minas Anor). However, Anarion recaptured the city, preventing Minas Ithil from being turned into an early Minas Morgul.

After Sauron was defeated in the War of the Last Alliance, a watch was kept from Minas Ithil to prevent the return of evil to Mordor. However, the Great Plague devastated the population of the city and in the following years its guard upon Mordor slackened.

The Nazgûl, led by the Witch-king of Angmar, the chief Nazgûl and most fearsome of Sauron's minions, returned to Mordor to prepare for Sauron's return. The forces of the nine Nazgûl laid siege to Minas Ithil. After a long siege, the city fell and was transformed into a bastion of evil. As a result, it came to be called Minas Morgul, which in Sindarin means Tower of Dark Sorcery. Minas Morgul stood upon the Morgulduin, a polluted tributary of the Anduin.

"A long-tilted valley, a deep gulf of shadow, ran back far into the mountains. Upon the further side, some way within the valley's arms, high on a rocky seat upon the black knees of the Ephel Dúath, stood the walls and towers of Minas Morgul. All was dark about it, earth and sky, but it was lit with light. Not the imprisoned moonlight welling through the marble walls of Minas Ithil long ago, Tower of the Moon, fair and radiant in the hollow of the hills. Paler indeed than the moon ailing in some slow eclipse was the light of it now, wavering and blowing like a noisome exhalation of decay, a corpse-light, a light that illuminated nothing."

The Nazgul

The Nazgûl (in the Black Speech); (English: Ringwraiths, sometimes written Ring-wraiths, also referred to as the Black Riders or as the Nine) were the dreaded ring-bound servants of the Dark Lord Sauron in Middle-earth throughout the Second and Third ages, and they dwelt during the Third Age in Minas Morgul.

The nine kings of men 

Once nine great Kings of Men, they were all given Rings of Power. The Nine took them and so became slaves of Sauron and later his chief lieutenants. After a time the effect of the rings left the kings spectral, invisible to all but those who could see into the wraith world (such as high elves or the bearer of the One Ring). 

The Nazgul

"Nine he gave to Mortal Men, proud and great, and so ensnared them. Long ago they fell under the domination of the One, and they became Ringwraiths, shadows under his great Shadow, his most terrible servants."

This is how the rings affected the nine kings of men

They often rode horses stolen from Rohan

Ringwraith

The witch-king of Angmar

The Witch-king of Angmar was the leader of the Nazgûl or Ringwraiths, and Sauron's second-in-command in the Second and Third Ages. 
Once a king of men, he was corrupted by one of the nine Rings of Power given to the lords of men and became an undead wraith in the service of Sauron.

After the first defeat of Sauron in the War of the Last Alliance, the Witch-King fled to Angmar, a kingdom he ruled for over thousands of years until he returned to Mordor to lead Sauron's armies in the War of the Ring. He was killed in the Battle of the Pelennor Fields by Meriadoc Brandybuck and Éowyn niece of Theóden at the end of the War.