Someone please correct me if i'm wrong..I'm not a LOTR expert. But I think Nazgul was the name given to the Nine Men who were given the Nine Rings of Power as mentioned in the verse of the Ring. They became corrupted by the Power of the Rings and transformed into Sauron's dark and deathless servants
I refer to Nazgullord as Nazguly and to Nazgulinthedark as naz 😄 there are several other naz' I think... and yey it's the short form of Nazgûl which litterally simply means "Ringwraith(s)"... Nazg is "Ring" (like in "Ash Nazg Durbatulûk" - "one ring to rule them all" or in the films "Durgbu Nazgshu" - "lord of the ring"😉, gûl means something like "evil spirit".
The actors? I only know Witchy's by name...