The Black Prism (Lightbringer #1) by Brent Weeks

Guile is the Prism, the most powerful man in the world. He is high priest and emperor, a man whose power, wit, and charm are all that preserves a tenuous peace. Yet Prisms never last, and Guile knows exactly how long he has left to live.

When Guile discovers he has a son, born in a far kingdom after the war that put him in power, he must decide how much he's willing to pay to protect a secret that could tear his world apart.