Headcanons: Of Rings And Kings, Slaves And Wraiths
((This is basically a timeline post, for those interested in the fall of the Nine Kings, according to our headcanons. As always, this headcanon applies only to my muse unless the other NazgulMuns say otherwise!))
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Second Age
Circa 1500: The first rings of power were forged by Annatar and Celebrimbor
Circa 1590: Three elven rings forged
1600: One Ring forged; Celebrimbor realizes Annatar's deceit
1693: war between elves and Sauron begins
2251: the ringwraiths first appear in the histories
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All that said, the basic assumption here is that all of the Nine were all living somewhere around 1500, when the first rings were completed. Sometime not long after the forging was complete, Annatar gifted the nine rings to the recipients of his choice, each for a specific reason. And the kings, too, accepted the rings for their own reasons. (Uvatha wished the power to finally forge a lasting peace among his people, for example.) But at the time the rings were given, no one yet knew that Annatar was Sauron--save those who had distrusted him from the start, of course. But at this point, there were even many elves who welcomed Annatar, and the mortal kings themselves had little concrete reason to distrust him or his gifts.
And it isn't until nearly 200 years after the Nine took their rings that the war between the elves and the now-revealed Sauron begins. During that time, the rings would have begun to whisper to their bearers, would have begun to warp them--or try to. This effort only increased in strength after the Ruling Ring was made, for that created the lasting connection and finally threw wide the conduit Sauron had designed between the rings and his own will. But by the time the One was created, it would likely have begun to become obvious to the Nine that their gifts were tainted. And afterward, that taint was clear.
Uvatha still fought, however. During that entire 200 years, and afterward, his will remained strong. As each of the others succumbed to the whispers of their rings, Uvatha held fast--though he felt it each time one of his brethren broke and fell. It took the complete destruction of all he held dear to finally shatter his will, and that drove him to the refuge of insanity--the doglike state into which he retreated making him a nearly perfect servant, despite his earlier rebellion.
Between 1693 when the war began and 2251 when the first Nazgul were sighted, the Nine were already serving Sauron in various ways. Each of them fell at different times, and the moment their wills broke they began to fade toward the fully wraith-like state.
But for a long time, they were still men to all outward appearances, thus explaining why they were not yet recognized as being the Nine. In this phase, they were slaves in the truest sense--chained (by magic) and made to do Sauron's will under threat of pain and punishment, but still individuals who were able to think relatively independently, and feel, and remember. But they still fought by Sauron's side because they literally could not rebel in any meaningful way, whipped and chained from within as they were. However, in this phase, Sauron would still be able to tell the Nine his will and then step back and let them accomplish it using their own talents.
But by 2251, all Nine were close enough to the fully-wraith stage of their existence to be recognized as such; their abilities had been enhanced in various ways, they had already been given the Nazgul Shriek and Black Breath powers, and their wills and individuality were frequently almost completely subsumed by Sauron's. (For more information about the Nine as wraiths, see my wraithcanons tag.)