The nine Ringwraiths (BS: Nazgûl) are the highest servants of the Dark LordSauron. They are former MannishKings of the Second Age, originally noble and stalwart rulers, but corrupted by the gifts the Dark Lord gave to them: the Nine Rings of Men. They have become the undead and undying slaves of their master, wielding many dark powers.
In Middle-earth: Shadow of War, some of the Nazgûl are re-imagined, seemingly each replacing one of the "original nine", some Nazgûl are identified as Helm Hammerhand, Yukâ and Riyâ, Isildur Elendilion and - ultimately - Talion. This is however in stark contradiction to the Legendarium in which clearly is stated that the Nine were nine Kings of men who were corrupted in the Second Age. They never died nor were destroyed and replaced by other, newer Wraiths.