From SuspireWiki

Duke is a Title of Tribute bestowed upon a vampire who is held responsible for conquering a city and establishing a strong base of power for lasting rule by the Invictus. A Duke is customarily given a permanent place in the Inner Circle of the city he or she has secured, as well as permanent personal rule over a domain within the city. The name of the city in question is appended to the title (for example, The Duke of Richmond). No more than one Invictus vampire may traditionally hold this title in a city’s history. Even Final Death does not free the title for conferring upon another, though in modern nights this practice is relaxed somewhat if the Invictus are forced to re-conquer a city following a prolonged period of absence. Thus, a few cities hold Kindred with titles such as “the Second Duke of Glasgow.” His Grace or Her Grace precedes the name of a Duke or Duchess in introduction.