Wan Xian literally means Ten Thousand Immortals. Allegedly, it's the original name of the Kuei-jin.
According to legend, the Wan Xian were the 10,000 heroes of the Middle Kingdom. They originally protected the Middle Kingdom from the depredations of the Yama Kings and other threats. Chosen by the August Personage In Jade, they were mortals who passed various tests and acquired the ability to feed off of chi, granting them great power. The Wan Xian were eventually corrupted by power and the Yama Kings, until they committed crimes too great to be forgiven, and they were cursed to become the Wan Kuei.
The story of the Wan Xian's fall is intended to echo the story of the Exalted, with the Wan Xian being some kind of Exalted - possibly Solar Exalted (in terms of power) or Terrestrial Exalted (in terms of number). The Wan Xian are further echoed in the modern era by Hunters, which are an intentionally constrained reformation of the ancient heroes.
- Kindred of the East Rulebook, p. 16