Episode 1.6 is about Otto the Great. Otto I was the King of Saxony, King of East Francia, and eventually, the Holy Roman Emperor. His re-establishment of the Empire set it on a path to essentially be a continuous empire for the next 1,000 years.
Timothy Reuter, Germany in the Middle Ages c. 800-1056
James Westfall Thompson, Feudal Germany
Karl Leyser, "The Battle of Lech. A Study in 10th Century Warfare", History
Karl Leyser, "Ottonian Government", The Journal of Historical Review
Gerhardt B Ladner, "The Holy Roman Empire of the Tenth Century and East Central Europe", The Polish Review