The Serpent Bride by Sara Douglass
And there are other dangers. For while Tencendor is gone, even its fall cannot destroy the Icarii. As the Tyrant of Isembaard reaches for glory, both StarDrifter SunSoar and his son, Axis, are pulled into the deadly dance of intrigue and sorcery. The DarkGlass Mountain—once known as the Threshold—is waiting, and as the Dark God Kanubai rises from his prison in exile, no one will escape unscathed."
Mass market, a nice copy!