The series began with the tagline "The science fiction decade of the decade begins here." A lofty goal for any author, but if there was someone who could pull off such a lofty goal it would be David Weber. Does he deliver though?
As a small introduction to the series, we go some 300 years into the future where humanity has spread to the stars, only to suddenly and horrifically discover that we are not alone. The genocidal alien race the Gbaba slams into humanity with all the force of an interstellar avalanche and one by one wipes out mankind's colonies until only Earth is left facing the invaders. Soon, Earth too dies.
But that is not the end, as a last desperate attempt to save the human race, Operation Ark, is launched to find humanity a new home where they can hide in safety and one day rebuild, and take the fight to the Gbaba. However, the leaders of Operation Ark have slightly different plans and essentially brainwash the colonists into believing they are now, and always have been, denizens of the planet Safehold and were put there by God, and the leaders of Operation Ark are the archangels who watch over them by entrapping them in the Middle Ages. Suffice to say some members of the operation disagreed with this, and in doing so they get wiped out by an orbital bombardment, becoming demons in the worlds mythology. However, they have an ace in the hole, a cybernetic avatar of a dead naval officer named Nimue Alban who adopts the persona Merlin to break the grip of the ruling theocratic body of the planet, the Church of God Awaiting.
Merlin chooses the mercantile nation of Charis and makes their enemies his enemies, and in doing so plunges the world into a holy war in a struggle for its very soul.