6 decades books

My name is Seraph. In a world that is filled with magic, science and arts of unbelievable kinds, I''''m relatively ordinary. But then I met them. A group of assassins that work the machines of the world behind closed curtains. And the most striking is their leader, the woman which is the greatest mystery in the world. Though to the world we are all dead, each of us has a story of their own. I wonder if I can find myself a home among these people who call themselves Valkyries and more importantly can I solve all the riddles that surround them?