Nine "proud and great"
For many years the nine used these rings, which gained them great wealth, prestige, and power.
However, the effect of the rings made their bodily forms fade over time until they had become wraiths entirely, and served only the Dark Lord!
The fear the Nine inspire is one of their greatest strengths.
The Nine cannot see during the day as mortals do; instead, they see shadowy forms.
During the night they see many signs and forms invisible to mortal eyes; it is at night that they are to be feared most.
They can smell the blood of living things that they desire and hate.
Their presence can be felt as a troubling of the heart, and they can more keenly feel the presence of others.
At all times, they sense the presence of the Ring and are drawn to it.
They are surrounded by an aura of terror, which affects all living creatures;
their aura (called the Black Robe Breath) can be toxic to those hapless enough to come near them.
Of course, their horrible cries put many a battle-hardened warrior to flight as well.
Some of the Nazgûl appear to be accomplished sorcerers and use magic to devastating effect.