This alternate history features a world in which Marcus Furius Camillus, a Roman soldier in a period of voluntary exile, steps on a snake while travelling to Ardea. He is subsequently poisoned and dies.
In OTL, during the Gaulish invasion of Rome, Marcus Camillus was the soldier who organised the resistance and succeeded in expelling the Gauls. The city of Rome would later go on to lead one of the greatest empires that ever existed.
In this timeline, however, without Marcus Camillus to lead the resistance, the Gauls successfully conquered and destroyed the Roman resistance, eliminating the Roman Empire before it even started. This drastically redefines power in the Mediterranean and, in effect, the whole world.