The Ringwraiths had no great physical power
against the fearless, but their greatest weapon was the unreasoning
inspired and which was enormously increased in darkness. They did not
scream but kept a more terrifying silence. The Ringwraiths were evil
Nùmenòreans or Sauronians.
What he (Tolkien) means by Sauronians I am not sure of. Breedings from Southrons and Numenoreans perhaps? Or just men/kings from countries under his dominion?
Reference: Letters of Tolkien