Judging from HP7, when the trio was in the tiger's den, Gellert. Grindelwald held off Voldemort for a long time, and Harry avoided watching or listening to their conversation. So, in the face of impending death, or what Dumbledore called "another great adventure," what did the former Dark Lord say to his successor during this time? What did he realize during so many years in prison? We can only guess.