A Dance of Blades. Book 2 of Shadowdance

It's been five long years since the city learned to fear... The war between the thief guilds and the powerful allegiance known as the Trifect has slowly dwindled. Now only the mysterious Haern is left to wage his private battle against the guilds in the guise of the Watcher - a vicious killer who knows no limits. But when the son of Alyssa Gemcroft, one of the three leaders of the Trifect, is believed murdered, the slaughter begins anew.
Mercenaries flood the streets with one goal in mind: find and kill the Watcher. Peace or destruction; every war must have its end. Fantasy author David Dalglish spins a tale of retribution and darkness, and an underworld reaching for ultimate power.