Courting Darkness (Courting Darkness duology)

From New York Times best-selling author Robin LaFevers comes Courting Darkness, a “sharp and breathless” (Kirkus Reviews) historical fantasy for fans of Leigh Bardugo and Game of Thrones. Set in the world of the His Fair Assassin series, this first in a new duology now has four starred reviews!
Death was only the beginning.
Sybella has always been the darkest of Death’s daughters, trained at the convent of Saint Mortain to serve as his justice. But in a desperate bid to keep her two youngest sisters from the family that nearly destroyed them all, she agrees to accompany the duchess to France. Surrounded by enemies, their one ray of hope is Sybella’s fellow novitiates, hidden deep in the French court years ago—provided Sybella can find them.
Genevieve has been undercover for so many years, she struggles to remember who she is or what she’s meant to be fighting for. Her only solace is a hidden prisoner who appears all but forgotten by his guards. When tragedy strikes, she takes matters into her own hands, even when it means forsaking the long-awaited orders from the convent.
Followed from alternating perspectives, Sybella’s and Gen’s paths draw ever closer. As their worlds threaten to collide, the fate of everything they hold sacred rests on a knife’s edge.