In the book Blue Blood, True Blood, Stewart wrote the Vikings were genetically infused, programmed and directed by aliens from Aldebaran.
It is generally known that:
The period between 750-1300 was the Viking era in Europe, whereby between 750-950 was the period of the big violent trips of the Vikings.
From Norway & Denmark they conquered Great Britain, France, Iceland, and parts of America and parts in Southern Spain, Italy and Sicilia.
From Sweden they went deep in Poland, Russia, to the Caspian Sea, Black Sea, Turkey and the Kaukasus.
When you have a geographical look on this, it's clear The Viking movements were a reaction against the emerging Roman Catholic Empire and Byzantium.
I've no idea what exactly caused the end of the Viking influences, or why the Aldebareans had leaved the Earth in the Middle Ages.