In Middle-earth, the Ringwraiths, also known as the Nazgûl or Black Riders, are neither living beings nor ghosts in the traditional sense. They were once powerful and mighty mortal men, but their souls became bound to the One Ring, which was created by the dark lord Sauron to dominate and control the other Rings of Power.
When they accepted the Rings of Power offered to them by Sauron, the nine men who would become the Ringwraiths were granted great power and extended life. However, as a consequence of wearing the Rings and succumbing to Sauron's corruption, their souls became bound to the will of the Dark Lord. Over time, their physical bodies faded away, and they became wraith-like and invisible to ordinary mortals. They exist in a shadowy and spectral form, making them appear as dark, hooded figures with terrifying abilities.
The Ringwraiths are powerful servants of Sauron, and their main task is to hunt for the One Ring and bring it back to their master. They are drawn to the Ring's power and can sense its presence when it is being used or carried nearby. The closer they get to the Ring, the more powerful they become, but they are also more susceptible to its influence and control.
So, while the Ringwraiths were once mortal men, their association with the Rings of Power and their subsequent subjugation to Sauron's will transformed them into spectral beings, neither fully living nor truly dead, but eternally bound to the fate of the One Ring and its master.