Eagle -- Semi-sacred standard of a legion; symbol of Roman power
Great southern king -- Augustus
Karl -- Warrior
Skald -- Bard
Southern men -- Romans
The battle -- Teutoburgerwald  in 9 CE in which Hermann (Arminius) and the Cherusci with allied tribes defeated Legiones XVII, XVIII, and XIX under Publius Quinctilius Varus during a great storm in the forest of Teutoburg
Thrall -- Slave
Thunderer -- Thor/Donar
Thusnelda -- Daughter of Segestes of the Cherusci
The Germans forgot Hermann and his deeds until the rediscovery of the Roman classical writers in the Renaissance . So there must have been a time when the memory was dimming, and I propose it was sometime between the time of Boniface and Charlemagne, perhaps in the middle of the 8th century. A minor German chieftan is host to Christian missionaries during Yuletide. His "great hall" is the main building of a farmstead, perhaps at one end of small vale in northern Germany.
The chieftain foresees and laments the passing of the Northern gods as well as the memory of Hermann the Cheruscan as Christian influence and intolerance wax.