But that isn't the only thing that's a bit outlandish about them. There's also the fact that they're Gaulish nationalists, and that many of their songs are about the heroic struggle for national liberation waged by Gaulish resistance fighters against the might of Imperial Rome- oh, about two thousand years ago.
In case that isn't eccentric enough, they also frequently sing in Gaulish, a language no one has spoken since sometime before the fall of the Roman Empire. It's the Celtic language once spoken in France and central Europe, just one generation removed from the Proto-Celtic language from which all the Celtic tongues were ultimately derived.
So there you have it. Heavy metal/Celtic folk music sung in a long-dead Celtic language about a long-lost rebellion for Gaulish independence against a long-vanished empire. If you don't understand what's cool about that, I'm not really going to be able to explain it to you.
Even though the Gaulish struggle, and more particularly the Helvetian struggle, went down in defeat a very long time ago, this band of Swiss musicians still carries the torch. Tremble, minions of Rome- the Helvetii have risen again!