Brennus, leader of the Celtic invasion of Greece

It’s the year 279 BC, Alexander the Great is dead and his generals have spent the last 40 years fighting each other in a vicious war of succession. When they were in their 20s, they followed Alexander to the end of the known world as his generals and companions, but these of them who are not dead yet, are now old kings of their own Hellenistic states. The Macedonian empire that a few decades ago stretched from the Danube to the Nile and from the Balkans to the Himalayas has fragmented into several successor states.
Brennus (who must not be confused with Brennus of the Senones, who sacked Rome a century earlier, and whose name some believe to be a title rather than a proper name) was initially one of the several Gallic leaders that invaded the Balkan Peninsula. But at the end he assumed command of the great expedition that penetrated deep into Greece.