The Ember Blade / Chris Wooding.

Empires rise, civilisations fall and one culture comes to subsume another. It's the way of the world... sometimes ways of life are improved, sometimes they are not. But the progression of change is huge and - usually - unstoppable. In this story, the Ossian way of life is fading and the Dachen way is taking its place and Aren is comfortable with that. Even when his parents are accused of treason he supports the establishment and maintains there's been some mistake... which is all it takes to get himself and his best friend arrested... Thrown into a prison mine they plan their escape - only to be overtaken by events when they're rescued, and promptly find themselves in the middle of an ambush. By the time they've escaped, they're unavoidably linked to Garric - their unwelcome saviour - and his quest to overturn the Dachen way of life. If they leave Garric now, they'll be arrested or killed by their pursuers. If they turn him in, Garric will kill them. If they stay with him, they'll be abetting a murderous quest they don't believe in. There are no good options - but Aren will still have to choose a path...