Alsara: Requiem and Revolution
Hauptstadt is the capitol of the Empire. Regency is a word referring to the rule of a Regent: a figure who is not the king, but standing in for one until a proper ruler can be found. You have heard the two used in conjunction several times in passing, but have yet to get any more information than that.
(The party has not yet encounterd any information concerning the Hauptstadt Regency)