Treason of the Long Knives

In three keels 

came the Saxons 

the mercenaries

to settle in the East

these faithless ones

broke their treaty

plundered Britain

sea to sea 

burning without cease

the whole of the island

and licking the western ocean

with a tongue of savage red. 

Vortigern, king of the Britons

invited Hengist and Horsa

the Saxon leaders 

to aid him in his conflict 

with the Picts & Scots

but this misjudgement

led to the killing of his sons

and the loss of Britain

to these Germanic brothers.

Leading the Saxons

these two came to Ebbsfleet

on the Isle of Thanet

but according to the tales

they betrayed Vortigern 

and the others 

in the Brad y Cyllyll Hirion

the treason of the long knives

a massacre of the Celtic chieftains

at a conference of peace

on Salisbury Plain.

Detail from Lambeth Palace Library MS 6 folio 43v illustrating an episode in Historia Regum Britanniae (c. 1136). Pictured above Vortigern sits at the edge of a pool whence two dragons emerge, one red and one white, which do battle in his presence.

https://en.wikipedia.org/wiki/Vortigern#/media/File:Vortigern-Dragons.jpg