In 387 BC (the old Roman chronology says 390), the Gaul tribe of the Senones defeated the Romans at the river Allia and entered the city, leading to what would become known as the first sack of Rome (relevant History of Rome podcast is here). The Roman leadership fled to the impregnable Citadel, but for months the city would suffer at the hands of the Gauls.
The Gaul’s leader was Brennus. My impression is that we know him only through Roman writers, which is to say we know almost nothing about him (some historians even dispute Brennus was in fact his name). But I like the legend the Romans tell about the price they had to pay to make the Gauls go away. When they presented Brennus with the thousand pounds of gold he demanded, the Romans complained that the scales supplied by Brennus to weigh the gold were inaccurate. The Gaul chieftain allegedly dropped is large sword on the weights, exclaiming Vae victis!, meaning the vanquished had no rights and should do as they are told.
There is no question the sack made a huge effect on Roman psychology. They wrapped the events in myth and stayed wary of the Gauls for centuries, in fact until Gaius Julius Caesar invaded gaul 300 years later. It’s easy to think the horrible fate of the Gaul leader Vercingetorix at the hands of ‘Caesar’ was the citizens of Rome finally taking their revenge on Brennus.