To Whatever End: Echoes of Imara Book 1 by Claire Frank

Veterans of a past war, Daro and Cecily left the politics and turmoil of their kingdom behind, making a home for themselves where they can live in peace. An unexpected attack fractures their quiet existence and although Cecily narrowly escapes with her life, Daro is taken captive. He awakens to a world where his captors exact complete control. Forced to wear a mask and given a number instead of a name, he endures crushing psychological torture, conditioning him to be an experimental subject in a madman’s bid to bend the laws of Wielding magic.
As Cecily turns to old friends and attempts to mend broken alliances in her desperate search for her husband, Daro struggles to hold onto the shattered pieces of his mind. No sacrifice is too great to bring Daro back, but as the days tick by, he may succumb to his captors, becoming another living weapon in a growing army of abominations.