From Start To Finish

The action of Poul and Karen Anderson's King of Ys tetralogy takes twenty five years. For seventeen of those years, Gratillonius is King of Ys. The fourth volume describes the remaining eight years. Gratillonius, remarried with two young children, is still active, now as the Duke of Armorica but unofficially described as "King."

Almost everything changes:

the sea destroys Ys;
civil war divides the Empire;
no usurper succeeds in restoring central control;
one usurper withdraws troops from Britannia;
Armorica, then neighboring regions, expel Imperials;
the Pagan Gods dwindle or withdraw;
the new God's power grows;
Paganism becomes persecuted witchcraft;
Gratillonius converts from Mithraism to Christianity;
Bishop Corentinus (St Corentin) exorcises the mermaid who had been the last vengeance of the Ysan Triad;
barbarians advance even into Italy;
feudal dependence of serfs on landholders or monasteries begins.

The world changed then as now.