You are the once-trusted bodyguard of the beloved Empress. Framed for her murder, you become an infamous assassin, known only by the disturbing mask that has become your calling card. In a time of uncertainty, when the city is being besieged by plague and ruled by an oppressive government armed with neo-industrial technologies, dark forces conspire to bestow upon you, abilities beyond those of any common man – but at what cost? The truth behind your betrayal is as murky as the waters surrounding the city, and the life you once had is gone forever.
http://www.dishonored.com/
Looks like a cross between a Steampunk HL2, Assassins Creed, and Bioshock.