According to the encyclopaedia of Arda:
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One of them, the second in rank after the Lord of the Nazgūl himself, was named Khamūl, and also known as the Black Easterling. This is the only one of the nine Nazgūl explicitly named by Tolkien
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I always assumed that the WitchKing ruled over Angmar after he became a ringwraith, rather than before. I don't know why.