here because it hardly counts as a ninth stage in Ythrian history but no less a figure than Dominic Flandry comments:
"'The sophont races will survive. In due course, they'll build fascinating new civilizations. Cultures of mixed species look especially promising. Consider Avalon already.'"
-Poul Anderson, A Stone In Heaven IN Anderson, Flandry's Legacy (New York, 2012), pp. 1-188 AT p. 75.
Flandry and Anderson hint at a ninth stage that would have been outlined if it had been possible to continue the Technic History indefinitely. Flandry means that the sophont races will survive the collapse of the Empire which will result from a cycle of rule by force, oppression, general unhappiness and revolts led by ambitious Imperial claimants.
Flandry has striven to ensure that certain planets, like Nyanza and Dennitza, will remain strong after the fall of the interstellar civilization. Dennitza also has a culture of mixed species. Avalon has been strengthened by:
its Founder, Falkayn;
its inter-species cultural synthesis;
its survival of the Time of Troubles;
its incorporation into the Domain of Ythri;
its successful resistance to the Empire.
Thus, although Avalon has not benefited from any interventions by Flandry, it will hopefully be among those planets that are destined to rebuild interstellar civilization after the Long Night.
I am trying to post about Flying Mountains. However, a comparison with the Earth Book has reopened an interest in the successive stages of the Ythrian history.