Here is robertp6165's introduction to the timeline:
“One period of history which has always been fascinating for me is the early Dark Ages in Britain, the period of the Anglo-Saxon invasions. I have always been especially fascinated by the shadowy figure of King Arthur (reigned ca. 490-ca. 517 A.D.). I read a really interesting book called KING ARTHUR: A MILITARY HISTORY, by Michael Holmes.
In this book, Holmes postulates that Arthur was a historical Celtic High King (similar to Brian Boru in Ireland), who, in a campaign which culminated in the victorious Battle of Badon Hill, halted the advance of the Anglo-Saxon invasion for almost 20 years, and in some areas, actually rolled the invaders back. While there are many who would argue that Arthur never existed, there is good archaeological evidence to show that the Anglo-Saxon invasion was indeed halted and rolled back during this time period, strongly suggesting that Arthur, or some other figure playing the same role, existed. Holmes goes on to suggest that Medraut (Mordred) was a lieutenant of Arthur's who was placed in charge of the eastern marches of Arthur's kingdom, where he was charged with watching over the remaining Saxon invaders. Medraut instead allied himself with them in a bid to gain the High Kingship for himself, and in the Battle of Camlann (which Holmes places in 517 A.D.), both were killed. The British fell to squabbling among themselves, and the Anglo-Saxons were able to make a comeback, completing their conquest of most of what is now England by 584 A.D.
Let's assume that Holmes is correct, and that Arthur was indeed the Romano-Celtic High King of Britain, and that he was responsible for rolling back the Anglo-Saxon invaders. Let's assume that Medraut rises in rebellion, as in OTL, however, let us also assume that Arthur is able to defeat Medraut's forces and survive the battle. He has a strong son, and establishes a dynasty which over several generations manages to consolidate its rule over Romano-Celtic Britain. The Anglo-Saxons never manage to expand out of their initial bridgeheads into the interior of the island. Let's assume that the Scots still cross over from Ireland, and found their kingdom of Dalriada, where they compete with the Picts for supremacy in what is now Scotland. What would be the effect on history of a strong, unified Romano-Celtic kingdom in Britain?.”
So, let's cut to the chase
Well, to judge by the timeline, the butterflies would be enormous. How about no Crusades? A longer-lived Carolingian dynasty in France? Or an earlier Reformation? How, you ask?
To find out, read it here!