well if you watch advent children complete the white light is there and zack is standing behind him and ulap says "i am your living legacy" then he defeats sephiroth
You need to play Crisis Core to understand that...after ulap defeated Sephiroth..he and zack woke up in a laboratory where dr.Mojo planted Sephirots sells to zack and ulap and made a new Project...it's kinda weird but u need to play crisis core to understand...anyway excelent game...the whole FINAL pantasiya 7 :D