Among the rest, there was in the city of Bangor an abbey so noble, and the number of monks in it was so great, that when the monastic body was divided into seven sections, each with its prior duly appointed, none of the sections had less than three hundred monks, all of them living by the labour of their hands. Their Abbot was called Dinoot and he was a man remarkably learned in the liberal arts. [XI.12]

Geoffrey introduces BANGOR as the site of a monastery whose abbot refuses to submit to the authority of Augustine. In the next chapter, the monastery is destroyed by the army of Ethelfrid, King of the Northumbrians:

A huge army was assembled and ordered to march to the city of Bangor and destroy Abbot Dinoot and the other churchmen who had scorned Augustine. They accepted Ethelbert's orders, collected an enormous army together and set out for the land of the Britons. They came to Chester, where Brochmail, who was in command of that city, awaited their coming. A great number of monks and hermits from all the different territories of the Britons, and especially from the city of Bangor, had sought refuge in Chester, so they could pray there for the people’s safety. Armies were drawn up on both sides, and Ethelfrid, King of the Northumbrians, joined battle with Brochmail. Brochmail stood firm against him, although his force was smaller. In the end, however, Brochmail abandoned the city and fled, but only after inflicting enormous losses on the enemy. When Ethelfrid occupied the city and discovered the reason why these monks whom I have mentioned had come there, he immediately let his soldiery loose against them. That same day twelve hundred monks won the crown of martyrdom and assured themselves of a seat in heaven. After this the Saxon tyrant marched to the town of Bangor. When they heard of his mad frenzy, the leaders of the Britons came from all directions to oppose him: Blederic, Duke of Cornwall; Margadud, King of the Demetae, and Cadvan of the Venedoti. Battle was joined. They wounded Ethelfrid and forced him to flee. They killed so many of his army that some ten thousand and sixty-six died that day. On the side of the Britons there died Blederic, Duke of Cornwall, who commanded the others in these wars. [XI.13]

The story of the Bangor massacre is taken from Bede, but Tatlock writes that Bede located the monastery in question in Flintshire, in the northeast of Wales: it is marked on this map as BANGOR ISCOED. It seems that Geoffrey is identifying the location with the see-city of Bangor, however: he repeatedly calls the location a city, and he had just referred to the death of Daniel, bishop of Bangor [XI.3] (p. 65). Bangor is, the Cathedral site points out, the oldest see in Britain.