Brimstone

"Sariel, a fugitive slave, is running from the desert mines, and from an Empire that is hungry for a new and mysterious metal that the alchemists call brimstone. In a moment of mercy and lust, Ava Sandrino, herself a knight fallen from grace, shelters Sariel from his pursuers, and in the light of the moon, he speaks to her of a door. It is a gate that opens into a world that lies beyond the Northern Dark, over the edge of their farthest horizon. There, paupers rub shoulders with princes, and there are riches to be had by those with the will to seize them. Swayed by Sariel's tales of strange oceans and distant stars, and tired of a past that holds her down, Ava joins him on the trek to the kingdom where souls are remade. Together, they must cross a borderland that is the domain of magicians, the humans that serve them, and the One O'clock King: a faceless despot who guards the crossroads of worlds."--