Epic of Serinor: Time of Chaos
The shadowkin are not a coherent race in the same sense as the other nine races of Serinor: they are the undead of Serinor. Originally hailing from all the living races, they are hated by the pious humans, but respected and feared by those who have come to terms with their origin. Their subterranean cities of Aphrosta and Glimbor are little more than massive tombs, home to the large numbers of animate corpses and disincarnate spirits that make up the race of the shadowkin.
At the end of the Age of Heroes, the group of heroes responsible for the sealing of the hero shrines won the trust of the shadowkin and convinced them to reveal the secret of their origins: they were the husks of all past heroes, unable to pass on while the hero shrine sustained them, even as their body decayed around them. The end of the Age of Heroes destroyed most of the older shadowkin outright, and those that remained had to turn to dark magics to sustain their undeath and perpetuate their kind.
Coinciding with this turn to magic to sustain their race was an unexpected branching of their numbers. While some altered by their rituals retained their sentience and memories, most became mindless, soulless thralls. So while the terms shadowkin and undead are used interchangably in common parlance, shadowkin refers specifically to those whose souls have been tainted by shadow but still remain themself. Undead are those shadowkin whose soul was lost completely in the transformation. Lukosian scholars theorize that only those with a Hero Soul can survive the transition to undeath with their soul intact, a theory supported by the fact that, before the Time of Chaos, only those with a Hero Soul became shadowkin at all.