voivode vs warlord

voivode

noun
  • A local ruler or official in various parts of central and eastern Europe, especially early semi-independent rulers of Transylvania. 

  • An administrative chief in modern Poland. 

warlord

noun
  • A local ruler or bandit leader usually where the government is weak. 

  • A high military officer in a warlike nation. 

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