Gideon The Ninth by Tamsyn Muir

The Emperor needs necromancers.

The Ninth Necromancer needs a swordswoman.

Gideon has a sword, some dirty magazines, and no more time for undead bullshit.

Brought up by unfriendly, ossifying nuns, ancient retainers, and countless skeletons, Gideon is ready to abandon a life of servitude and an afterlife as a reanimated corpse. She packs up her sword, her shoes, and her dirty magazines, and prepares to launch her daring escape. But her childhood nemesis won't set her free without a service.

Harrowhark Nonagesimus, Reverend Daughter of the Ninth House and bone witch extraordinaire, has been summoned into action. The Emperor has invited the heirs to each of his loyal Houses to a deadly trial of wits and skill. If Harrowhark succeeds she will become an immortal, all-powerful servant of the Resurrection, but no necromancer can ascend without their cavalier. Without Gideon's sword, Harrow will fail, and the Ninth House will die.

Of course, some things are better left dead.