Dark Age Memory -- Arcana

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 [1] 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

Dark Age Memory -- Inspiration

The Germans forgot Hermann and his deeds until the rediscovery of the Roman classical writers in the Renaissance [2]. 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.

[1] An excellent article on Teutoburgerwald by Jona Lendering may be found at Livius. See also Fifteen Decisive Battles of the World by the exuberant Victorian Edward Creasy.
[2] Though some think that the Siegfried/Sigurd sagas do remember Hermann as their hero.