Night Masks by R. A. Salvatore

R.A. Salvatore's The Cleric Quintet tells the tale of the scholar-priest Cadderly, who is plucked from the halls of the Edificant Library to fulfill a heroic quest across the land of Faerun.
The assassins of the Night Masks stalk the shadowy streets of the city of Carradoon, led by the sinister killer known as Ghost. Cadderly struggles against the dreaded Chaos Curse in these dark, evil-haunted alleyways, with a silent death around every corner.