The Germanic tribes which inhabited western and central Europe in the Dark Ages.
In the story of Meriadoc, the Emperor of the Alemanni, at war with King Gundebald of the Land From Which No One Returns, employs the services of Arthur's Meriadoc. Meriadoc rescued the Emperor's daughter from Gundebald, and planned to marry her, but he was betrayed by the Emperor, who wished to use his daughter to seal a truce with the King of Gaul.
Meriadoc escaped from his prison, slew the Emperor, and married his daughter.
In history, the Alemanni were defeated by King Clovis of the Franks in AD 506. The author of Meriadoc may have drawn on this history for his Emperor of the Alemanni.