Who Did You Say They Are?
The Alemanni, or Alamanni if you prefer the English spelling.
You're wondering why you haven't heard of them if they were such a thorn in Rome's side (and they were). Well, actually you probably have. Their name survives in the French and Spanish words for Germany: Allemagne, and Alemannia, respectively.
The reason they didn't persevere to become a dominant force post ancient times is a little complicated. The Franks became the dominant force in Europe after the collapse of the Roman Empire in the fifth century AD. They were ready to step into the political, economical, and social void left by the contracting Roman influence. One of the reasons was they were willing to mold their society on Rome's example. They couldn't reproduce the brilliance of Rome -- the ensuing centuries were called the Dark Ages -- but they managed to hold things together until the Renaissance.
However, the Alemanni were always fiercely independent of Rome. Some of this attitude can be traced back to the treatment they received from the Roman emperor Caracalla at the beginning of the third century AD. He nearly destroyed the tribe through treachery and deceit, and apparently they never forgot or forgave. They even sided with the Huns during their rampage through Europe in the fifth century AD.
As the Franks rose to power they recognized the independent nature of the Alemanni. The Frankish king Clovis subjugated them in battle near the end of the fifth century. They became part of the Frank empire but still under home rule, as it were. When the Alemanni began to agitate for independence in the eighth century, their nobility, over a thousand men, were called to the Frank court, arrested, and executed. Rule over the Alemanni homelands was given to Frankish dukes.
The Alemanni retreated into themselves. Their dialect of German is still spoken, especially in the Black Forest area of Germany and also in Switzerland, Austria, and the Alsace region of France, . They have started to rediscover their proud history. There has been an increase in archaeological digs, and road signs in their traditional homeland are in both German and Alemanni.