Finished the book, and Cthulhu it is great!
Imagine a world parallel to ours, mired in the Middle Ages, around the time of the great schism between Rome and Constantinople (except it's not exactly those same cities).
The Holy Church (something something not Catholic, but, hey, just as oppressive and nasty, complete with Holy Inquisition) is convinced they have found the rightful heir to the throne of the Eastern Empire. And they put together a team of expandables to take her back home...