Reminds me of the Cimbri and Teutons, who scared the Romans so bad, they coined the term "Teutonic fury" to describe their fear. They defeated the Romans completely twice (one of the times Rome fucked up an ambush and got themselves slaughtered), and on the third encounter, Rome finally defeated them but the women killed all their children and then themselves.

... So they showed up seemingly out of nowhere, wiped out two Roman armies, then all died or killed themselves, denying the Romans any sort of trophy or compensation for their (very considerable) trouble. And that was just two tribes.