The nine Nazgûl are the highest servants of the Dark Lord. They are former Mannish Kings 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.
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 or were destroyed and replaced by other, newer Wraiths.