Harbinger (The Harrows, #1) by C. F. Bellairs

Within polished stone walls that more resemble a giant's broadsword driven into the earth than a true keep, lies are the building blocks of discussion, and the infamously-immortal Harrowlord rules with impunity, challenging each of his sons to lethal duels when they turn twenty-one. One of his bastards, Willem, is a Harrow idealist - but has never seen eye-to-eye with his father all the same. However, that may change when the royal bastard is entrusted with a special task: rooting out and beheading a traitor whose ideals he secretly embodies. As Willem struggles to understand his father's web of deceit, an ancient evil lingers at the precipice of his understanding, eager to flourish once more.