The Pagans of the White Dragon (Saxons & Angles) invasion of Britain as told by a Welsh monk named Nennius who lived in the 8th century.
The heathens became firmly incorporated, and were assisted by other foreign Pagans after landing in Britain in the year 449; for The king of the Britains Vortigern was their friend, on account of the daughter of Hengist, whom he so much loved and that the pagans had helped Vortigern and his father before him defeat the Picts, that no one durst fight against the White Dragons — in the mean time the White Dragons soothed the imprudent King Vortigern, and whilst practising every appearance of fondness, were plotting with his enemies. And let him that reads understand, that the Saxons were victorious, and ruled all Britain, not from their superior prowess, but on account of the great sins and failures of the Britains.
After the death of the old king, Hengist being strengthened by new accessions, collected his ships, and calling his leaders together, consulted by what stratagem they might overcome Vortigern and his army; with insidious intention they sent messengers to the king, with offers of peace and perpetual friendship; unsuspicious of treachery, the monarch, after advising with his elders and smoothed over by his wife Rowena (daughter of Hengist), accepted the proposals.
Hengist, under pretence of ratifying the treaty, prepared an entertainment, to which he invited the king, the nobles, and military officers, in number about three hundred; speciously concealing his wicked intention, he ordered three hundred Saxons to conceal each a knife under his feet, and to mix with the Britains; and when, said he, they are sufficiently inebriated, cry out “Nimader sexa,” then let each draw his knife, and kill his man; but spare the king, on account of his marriage with my daughter, for it is better that he should be ransomed than killed.
The king with his company appeared at the feast; and mixing with the Saxons, who whilst they spoke peace with their tongues, cherished treachery in their hearts, each man was placed next his enemy.
After they had eaten and drunk, and were much intoxicated, Hengist suddenly vociferated “Nimader sexa,” and instantly his adherents drew their knives, and rushing upon the Britains, each slew him that sat next to him, and there were slain three-hundred of the nobles of Vortigern. The king being a captive, purchased his redemption, by delivering up the three provinces of East, South, and Middle Sex, besides other districts at the option of his betrayers.
The (so called) Saint Germanicus, followed Vortigern to his citidel where he had retreated with his wives, and with his clergy, fasted and prayed to the Lord, three days, and as many nights. On the third night, at the third hour, fire fell suddenly from heaven, and totally burnt the castle in punishment for allowing heathens to take over Britain. Vortigern, the daughter of Hengist, his other wives including his own daughter whom he had a son with, and all the inhabitants, both men and women, miserably perished: such was the end of the unhappy king Vortigern, Others assure us, that Vortigern was then hated by all the people of Britain forever more, as having received the Saxons.
The moral of this story is to be wary of who you make friends with, especially if they offer you their pretty and devious daughters hand in marriage!
Probably an invention of the Welsh this story, it seems far-fetched. Probably the Germanic assimilation in the East happened slowly and without any extreme violence.