Ghost Omens (World of the Ghosts short story)

Caina Amalas is a nightfighter of the Ghosts, one of the Emperor's elite spies and assassins, and a bitter enemy of the magi. Claudia Aberon was once a proud sister of the Imperial Magisterium, but is now a desperate fugitive, sheltering from her father's wrath with the Ghosts.

And when rumors spread that a dead Emperor has returned from the grave, Caina and Claudia must work together to defeat the threat. 

Because the shadows of the past do not lie quiet...