overlord vs tetrarch

overlord

noun
  • A ruler of other rulers. 

  • A feudal lord directly owed rent or personal service by a tenant of subinfeudated land, a tenant's liege lord. 

  • Anyone with overarching power or authority in a given domain. 

verb
  • To rule over; to domineer. 

tetrarch

noun
  • A governor or ruler of a quarter of a country, especially of a fourth part of a province in or client state of Ancient Rome. 

  • An officer in charge of a fourth part of a phalanx in Ancient Greece. 

  • Each of the four members of a jointly ruling tetrarchy, notably of the Roman Empire since Diocletian. 

How often have the words overlord and tetrarch occurred in a corpus of books? (source: Google Ngram Viewer )