What does "when nine and nine meet nine" mean?
This is the explanation given by Hiroki Chiba (Scenario Writer) for the phrase:


  The number "9" is the last single-digit number so it symbolizes the
  “end”. The phrase is a unique abstract mythological expression which
  means the end of 3 signifies the end of the world (collapse of the
  cycle of death and rebirth). The unification of Orience, the end of 3
  countries, is one of the things that 9 and 9 and 9 implies.


Source