The Light of Eidon (Legends of the Guardian-King #1) by Karen Hancock

Abramm Kalladorne has dedicated the last eight years of his life to becoming worthy to touch and tend the Sacred Flames of Eidon, and he expects to be blessed for his devotion and sacrifice. But on the eve of taking the vows that will irrevocably separate him from the life he was born to—as fifth son of the king of Kiriath—he is betrayed by his spiritual mentor and sold into slavery by his brothers.
Swept along by the winds of a new destiny, Abramm is forced to compete in the gladiatorial games of his new masters. When the oppressed masses rally around his successes, he discovers his suffering has molded him into something greater than he ever thought possible—to serve a purpose he never imagined.