Asunder by Kerstin Hall

We choose our own gods here.

Karys Eska is a deathspeaker, locked into an irrevocable compact with Sabaster, a terrifying eldritch entity—three-faced, hundred-winged, unforgiving—who has granted her the ability to communicate with the newly departed. She pays the rent by using her abilities to investigate suspicious deaths around the troubled city she calls home. When a job goes sideways and connects her to a dying stranger with dangerous secrets, her entire world is upended.

Ferain is willing to pay a ludicrous sum of money for her help. To save him, Karys inadvertently binds him to her shadow, an act that may doom them both. If they want to survive, they will need to learn to trust one another. Together, they journey to the heart of a faded empire, all the while haunted by arcane horrors and the unquiet ghosts of their pasts.

And all too soon, Karys knows her debts will come due.