In German, "Kaiser" means Caesar or Emperor. The title was used to refer to rulers of German territories under several different empires, including the Holy Roman Empire and the first German Empire (the Deutsches Kaiserreich).
While all the other "D/D/D" monsters refer to very specific figures (and events), "Kaiser the Conqueror" is unique in that it is the most vaguely named member of the archetype (as there have been many "Kaisers"). However, of those figures crowned "Kaiser" the one who could most probably be deemed a "conqueror" would be Charlemagne, the first ruler of the Carolingian Empire which included all of modern France and Germany.
Alternatively, the "Kaiser" this card is modeled upon might be Otto I, the creator and first ruler of the Holy Roman Empire– which united all of the German tribes into one kingdom.