City of Darkness by Karina Halle

When mortal Hanna agreed to marry Tuoni, the God of Death, in exchange for her father’s life, the last thing she expected was to fall for the morally grey anti-hero. But it’s not only her heart that’s in danger now. As she navigates her new role as the Goddess of Death in the spooky and macabre underworld of Tuonela, Hanna discovers that Death has many enemies in his kingdom, none more powerful than his half-demon ex-wife, Louhi. With an uprising of Old Gods at her fingertips, and skeleton armies ready to do her bidding, Hanna and Death must work together to make sure the underworld isn’t lost forever, even if it means losing each other and all they hold dear.