The Kelmoren are not a distinctive race, instead they are a collection of life forms which have been the subject of a dark ritual. In the case of intelligent beings they are or were people who have dedicated themselves to the service of the dark god Sharkan and his allies. They are or were one of Sharkan's most useful types of servant, being able to operate normally amongst the other peoples of the world.
Externally the Kelmoren are indistinguishable from the species they have been derived from, for example a human Kelmoren externally at least remains human. According to legend however they are different in one important respect, they are a form on undead, possibly a type of lich? The Kelmoren do not have hearts within their bodies. The ritual which creates a Kelmoren somehow removes the heart and gives it to the god Sharkan. The "life" of the Kelmoren is then in Sharkans keeping.
Unlike "normal" undead the Kelmoren cannot be "turned" as they seem to have not yet crossed the final freshhold into unlife. As a consequence they are hard to "kill" as the connection between the Kelmoren and Sharkan must be severed. Even totally destroying a Kelmoren's body will not destroy it if this link is intact, the Kelmoren will simply regenerate elsewhere. If the link is severed then the Kelmoren is slain immediately and it is said, the goddess of Death will want to have a little "chat" with the Kelmoren that has finally come before her.