The Sphere of Secrets by Catherine Fisher
Every thing that could go wrong does. Seth caves in to blackmail while Alexos plans a journey to the Well of Songs...a place where no one returns from. With every thing at stake and nothing left to lose they set off on a quest that is sure to come to bring destruction.
Alexos the child/god/ruler, Seth the scribe, Oblek the warrior/now old, Jackel thief/guide, along with his partner Fox. The only hope they have is an ancient map carved into a silver sphere that they must follow to the end of the world to enable them to make things right and save two worlds.
The question is can they do it? Each of them have their own hidden agenda and do not care what happens to any of the others. Treachery, hardship, danger and deceit may keep them from reaching their common goal. And so they search, for the answers, for the stars, for the solution that could save them all from certain destruction.