Brennus (Civ4)/Civilopedia

Other than that he was a fearsome and successful warrior, little is known about the Celtic chieftain named Brennus. In 279 BC, he led an army of Celtic warriors on an invasion of Macedonia and northern Greece. They turned the Greek defense at the pass of Thermopylae and sacked the famous Oracle at Delphi - Brennus was wounded in the fighting. From there, the Celts penetrated into Epirus (roughly modern Albania) and looted the treasures stored at the temples of Zeus at Dodona and Olympia. After those triumphs the Greek resistance stiffened. Brennus retreated to Macedonia, where he died of his wounds. Without the leadership of their chieftain, the Celtic warriors split up and went in separate directions. Most of them returned to their homes in Gaul along with the treasure they had pillaged, while a few others headed east and settled in Asia Minor. This group came to be called the Galatians, and remained distinct from their surrounding neighbors for at least two centuries.
The name "Brennus" is more likely a title than a proper name - in fact the same title was earlier given to another Celtic chieftain who sacked Rome in 390 BC. It is very likely that the Welsh word "Brenin," meaning king, derived from that title.