Twilight Falling - The Erevis Cale Trilogy

Newly dedicated to the Mask, an assassin-turned-butler returns to the shadowy lifestyle he once left behind 
 
Over one decade ago, the Night Knives placed spy Erevis Cale into the wealthy Uskevren household, tasking him with gathering intel that would tarnish the family's name. But as Cale's relationship with his employers deepened with time, he grew tired of serving two masters—and ultimately sided with Thamalon, the Uskevren patriarch.
 
Now, Thamalon lies dead, and the sun has set on Cale’s service to the family. But just when his future seems more uncertain than ever, a letter arrives from a prominent figure from his past. By day’s end, Cale has dedicated himself to a new master—one who is beyond the petty accumulation of wealth. After all, what is gold to one who trades in souls?