Beneath Ceaseless Skies - Laws of Night and Silk by Seth Dickinson
This is how an abnarch is made. This is the torment to which Kavian gave up her first and only born. The wizards of the Paik Rede, dam-makers, high rulers of isu-Cter, seal a few of their infants into stone cells. They grow there, fed and watered by silent magic, for fifteen years. Alone. Untaught. Touched by no one. And on nights like these, their parents decant them for the war.