Night of the Long Knives
ca. 450 CE, Salisbury Plain, UK.
Oddly, there is an earlier "Night of the Long Knives" than that of the Nazis. According to Geoffrey of Monmouth (not the most reliable of sources, mind you), Vortigern had attempted a peace treaty with the Saxons that he had originally paid as mercenaries in order to fight the Picts and Scots. However, the Saxons had taken to, well, taking the land from the Britons. Vortigern set up a meeting between the Saxons and British chieftains at a monastery out on Salisbury Plain.
Unfortunately, the Saxons came armed, and slaughtered 460 British chieftains. The ensuing violence was called (at least by Geoffrey) "The Night of the Long Knives."
Later, Ambrosius Aurelianus defeats Vortigern and becomes king. He consults Merlin on constructing a proper monument for the slain cheiftains. Merlin suggests removing stones in Ireland, the King's Ring from Mount Killarus, and bringing them to England.
This is the founding of Stonehenge, at least as far as Geoffrey was concerned. Of course, by 475 CE or so, Stonehenge had already been there around 2500 years or so.