Lynx sent him there in Fort Dragonia. This is a part of the plot which I never fully understood. Why didn't Lynx just kill him? He seemingly sends him there so that he has the possibility of living. Then later on, after he regains his body and enters the Sea of Eden, Lynx/Dark Serge/FATE apparently had been waiting for him to get there the entire time. FATE gives the little speech about "blah blah blah I've been using you this whole time" and then proceeds to try to kill them like it should have done in the first place.
I can't figure out FATE's logic here. If FATE is killed, the Dragon Gods are released and this is not what FATE wants. FATE only needs Serge's body to get into the Sea of Eden and regain control over the Frozen Flame and that's it. FATE uses Kid to lure Serge there, and then tells him the grand master plan only to try to kill him afterwards? What the hell was the point of that?
But all that is besides the point. Back on the topic of the Mystics, we can safely say that Sprigg originated in the real world, and that the other Mystics probably originated in the real world too. The first time we see them is in 12,000 BC, after humanity has made contact with the Frozen Flame. There is probably a connection there.