Brennus (or Brennos) (died 279 BC at Delphi, Ancient Greece) was one of the Gaul leaders of the army of the Gallic invasion of the Balkans. While invading the Greek mainland he managed to momentarily reach as far south as Delphi in an attempt to loot the rich treasury of the sanctuary of Apollo. His army suffered a devastating defeat at Delphi, he was heavily injured during the battle and committed suicide there. His militarily inexperienced army was forced to a continuous retreat by the tactical attacks of the Greek city-states and was cut down to a remaining band that fled from Greece.
In 280 BC a great army, comprising about 85,000 warriors, coming from Pannonia and split in three divisions, marched south in a great expedition to the Greek mainland against Macedonia and then further south to central Greece as far south as Delphi during a failed and short-lived campaign against the Greek city-states. The division led by Brennus and Acichorius moved against the Paionians.
Some writers suppose that Brennus and Acichorius are the same person, the former being only a title and the latter the real name.
The other two divisions were led by Cerethrius and Bolgios, moving against the Thracians and Triballi, and against the Macedonians and Illyrians, respectively.
Brennus is said to have belonged to an otherwise unknown tribe called the Prausi. These Gauls had settled in Pannonia because of population increases in Gaul, and sought further conquests.
Source: Wikipedia, "Brennus (3rd century BC)", available under the CC-BY-SA License.