The Nine were a group of nine men turned to wraiths by the rings of power.
They are all avaliable in the rise of the witch king expansion with the witch king , Khamul , Morgomir And the black riders hero horde.
On the other games various mods let you control the nine.
They were once great kings of men until sauron the deciever gave them each an ring of power. Over the years they turned to wraiths to server sauron.
After the witch kings demise there deaths followed shortly.
Nazgul are powerful heroes that can defeat battalions with ease