In 390 BC, when his father was pontifex maximus, he and his two brothers, Numerius and Caeso, were sent as emissaries to a Gaulish army besieging Clusium.
Instead of entering into negotiations, however, the three Fabii gathered their forces and aided the citizens of Clusium in an attack against the Gauls, in which Quintus Fabius himself was said to have killed one of the Gaulish leaders.
[1] Outraged, the Gauls demanded that the senate hand over the three brothers for violating "the law of nations".
Further incensed, the Gauls marched on Rome, defeated the Roman Army in the Battle of the Allia, and sacked the city.
[2] Quintus Fabius Ambustus was played by Tony Kendall in the 1963 film Brennus, Enemy of Rome.