The Undead race was a collective result of heterodox soul paths, walked by cultivators who rejected the limitations of flesh, lifespan, and orthodox spirit Qi.
Any being that cultivated an Undead Soul was classified as part of the Undead Race, regardless of their original species, bloodline, or origin.
At its core, the Undead Race pursued existence beyond death.
Unlike living cultivators, whose lives are anchored to flesh, or orthodox soul cultivators, who still rely on their fleshy bodies and bloodlines, Undead Race cultivators bind their existence directly to their soul persistence.
By doing this, as long as their Undead Soul remained intact, they could not truly die. Moreover, an Undead Soul was a corrupted yet stabilized soul form that existed between life and death, neither reincarnating nor dissipating.
