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