Before the Point of Divergence
Only days before the death of King Edward the Confessor of England on 5 January 1066, he briefly woke up from his apparent coma and entrusted the land of England to his loyal subject Harold Godwinson, Earl of Wessex. However, once the Witenagemot officially chose him as the true King and was crowned at Westminster Abbey the same day as the Confessor's death, opposing claimants to the throne became a significant issue.
Only weeks after the coronation, Duke William of Normandy, a distant relative of the previous King and supposedly vowed by Godwinson himself after a shipwreck near his land that he would swear fealty and support him in his claim to England, heard of the coronation and immediately started raising militia to claim the throne. After the Pope blessed William's cause and persuaded William's own noblemen to join him in his endeavour, Godwinson immediately raised his army throughout the country and assembled them all on the Isle of Wight, preparing for an eminent invasion. But, he later disbanded the army in London after seven months of no apparent threat.
On 8 September 1066 however, King Harold Hardrada of Norway along with Godwinson's traitorous brother Tostig landed at the mouth of the River Tyne in northern England. The Norwegian armies easily defeated the local Earls Edwin of Mercia and Morcar of Northumbria at the Battle of Fulford on 20 September. But, they were successfully defeated by Godwinson's own army in the Battle of Stamford Bridge on 26 September, with both generals being slain in the process.
On 27 September 1066 the Normans finally landed near Pevensey on the coast of Sussex. Godwinson's army ran a forced march all the way down to that area to intercept William's army. The two armies then finally battled on Senlac Hill on 14 October near the town of Hastings in which Godwinson and his two brothers were killed, decisively winning the battle and England itself for the Normans.
William was then crowned King William I, known mostly by the common Anglo-Saxon people as just "the Conqueror" on Christmas Day, 1066 in Westminster Abbey where he would reign until 1087, with his youngest son William II Rufus succeding him. However, there was still one Anglo-Saxon claimant to the English throne...
Point of Divergence
And that claimant was Edgar Ætheling, grandson of previous King of England Edmund Ironside. Although King William I had him in custody in Normandy after the conquest, he had fled with his mother and sisters to the Kingdom of Scotland until 1072, when William's men invaded Scotland and demanding the expulsion of Edgar and his family elsewhere.
The family then moved to the court of Count Robert of Flanders, an enemy of the Normans. In Flanders, the Ætheling had gained support by Robert and his noblemen, who were supposedly bribed and convinced that the marriage between William and Matilda, Robert's sister, was a marriage of "corruption, and downright damnation." In March 1089, the Ætheling and his loyalist men officially declared themselves hostile to the Norman Kingdom of England and on 14 April 1089, his men landed near Ipswich, England and decisively defeated the local militia in the Battle of Ipswich on 17 April. They then marched near London, winning a Pyrrhic victory against William Rufus's army in the Battle of Westminster on 22 April.
William Rufus later surrendered control of the Crown on 1 May, and the Ætheling was crowned in Westminster Abbey on 3 May.