The Death of the Necromancer — Martha Wells

Nicholas Valiarde is a passionate nobleman with an enigmatic past. Consumed by thoughts of vengeance, he is also the greatest thief in all of Ile-Rien, stealing jewels from wealthy nobles to finance his quest: the murder of Count Montesq. Montesq orchestrated the execution of Nicholas's beloved godfather on false charges of necromancy--the art of divination through communion with the dead--a practice long outlawed in Ile-Rien.

But a series of inexplicable, even fatal events interrupts Nicholas's mission. Children vanish, corpses assume the visages of real people, mortal spells are cast, and traces of necromantic power unused for centuries are found. When a spiritualist unwittingly leads Nicholas to a decrepit mansion, the monstrous nature of his peril finally emerges in harrowing detail. Nicholas and his compatriots must destroy an ancient and awesome evil. Even the help of Ile-Rien's greatest sorcerer may not be enough, for Nicholas faces ...a woefully mismatched battle--and unthinkable horrors await the loser.