Gwisin

Gwisin (I may toy with the name a bit) is a Korean term for "ghosts" who died unmarried and therefore are unhappy with their death. I read a bit about them here, but there's a lot more information on other sites, including Wikipedia I think.

They will be the "enemy" race in the story, a long forgotten race that will be making a comeback as the story begins, though the world will only be whispering about it at first. In the story they will not be ghosts, but humans who were unhappy in their life, to the point where they were contemplating suicide. When someone feels this way, they can be "recruited" into the ranks of the Gwisin. At first, they will look the same, but eventually they will become shadows, gray husks of their former selves. They have vague memories of their life, but are happy with the power they have and almost never look back.

Gwisin cannot recreate and therefore rely on this method to recruit. However, there is another rarer method, though it tends to create more powerful Gwisin. These humans actually seek out the Gwisin in search of power. This very rarely happens, as most humans fear the Gwisin, and those that seek them out often die before they ever find them. The Gwisin cannot find these individuals (as they "find" unhappy humans) because their drive for life is too strong. The normal method recruits sorry humans. The ones who seek them out are far from sorry or lost.

Once transformed, these "shadow people" are focussed warriors, intent on only expanding their power. The one trait they keep, besides the shape of their body, is their eyes, which retain their color and life. Because of this, humans believe that there is hope to return them their former lives, though none have ever heard of an actual retransformation.

This is a just a basic mythology I have begun to work with. It is far from complete but an important building block.