A Shadow on the Glass — Ian Irvine

"Once there were three worlds, each with its own human race. Then, fleeing from out of the void came a fourth race, the Charon. Desperate on the edge of extinction, they changed the balance between the worlds forever..."

THE TALE OF THE FORBIDDING
In ancient times the Way Between the Worlds was shattered, leaving bands of Aachim, Faellem, and Charon trapped with the old humans of Santhenar. Now Llian, a Chronicler of' the Great Tales, uncovers a 3,000-year-old secret too deadly to be revealed--while Karan, a young sensitive, is compelled by honor to undertake a perilous mission. Neither can imagine they will soon meet as hunted fugitives, snared in the machinations of immortals, the vengeance of' warlords, and the magics of powerful mancers. For the swelling deluge of a millennial war is rising, terrible as a tsunami, ready to cast torrents of sorcery and devastation across the land...