A soulkin, sometimes called a soul eater, is a person who is able to collect the physical part of a soul after something dies. Since the Manastorm when something dies it leaves behind a small crystal, that when touched by a non-Soulkin crumbles into dust. A Soulkin has the ability to collect this portion of a soul, generally trading it with supernatural beings called Merchant of Souls for money or increased powers.
Soulkin are extremely rare and often hide what they are because most societies see them as incredibly evil, selling souls for profit. In reality, the physical part of the soul is left behind because there is no use for it after something dies, the non-physical portion departs to wherever it’s supposed to go, leaving behind a small physical portion of itself that it no longer needs, however humankind and its subspecies are either not aware of this fact or refuse to believe it and tend to persecute and kill Soulkin when they are found out.
Soulkin also have the ability to, in limited form, return from the dead. Once killed, a Soulkin will rise from the grave, returned in an undead (read obviously undead) version of themselves (this is another reason they are hated and feared) and able to continue on. It is said that a Merchant of Souls are sometimes able to offer Soulkin rites to return them to their living forms.