David Weber'sA Mighty Fortress (Safehold Book 4) [Hardcover](2010)

"Young Cayleb Ahrmahk has accomplished things few people could even dream of. Not yet thirty years old, he's won the most crushing naval victories in human history. He's smashed a hostile alliance of no less than five princedoms and won the hand of the beautiful young Queen Sharleyan of Chisholm. Cayleb and Sharleyan have created the Charisian Empire, the greatest naval power in the history of Safehold, and they've turned Charis into a place of refuge for all who treasure freedom." "But their success may prove short-lived. The Group of Four, the clique that controls the Church of God Awaiting - and, through the church, most of Safehold - has decreed their destruction. Mother Church's entire purpose is to forever prevent the very things to which Charis is committed, and she will not rest until the Charisian abomination has been erased forever." "The Group of Four's first attempt failed, but they're determined to learn from their mistakes. Now they have no choice but to adopt some of Charis's innovations for themselves. Their mighty fleet, equipped with the new weapons, will soon be launched against Charis." "But there are still matters about which the Church knows nothing, including Cayleb and Sharleyan's adviser, friend, and guardian, the mystic warrior-monk named Merlin Athrawes. Merlin knows all about catastrophic battles against impossible odds, because he is in fact the cybernetic avatar of a young woman named Nimue Alban, who died a thousand years ago. As Nimue, Merlin saw the entire Terran Federation go down in fire, and slaughter at the hands of a foe it could not defeat. He knows that Safehold is the last human planet in existence, and that the technological stasis Mother Church was created to enforce will be the death sentence of the human race if it is allowed to stand." Mother Church's juggernaut is rumbling down on Charis, but Merlin Athrawes and a handful of extraordinary human beings stand in its path.