William the Conqueror
Edward the Confessor, last king of the House of Wessex, fell into a coma in 1065 and died in January of the following year having never named a successor. This necessitated the convening of the king's council known as the Witan, a collection of made guys, both secular and religious, to choose a new king. Having left behind no heirs, Edward's wife's brother seemed as good a choice as any and so was crowned Harold II.
Meanwhile, across the English Channel, Duke William II of Normandy saw his chance to make his move, ordering the manufacture of 700 boats and rallying a massive number of troops.
As luck would have it, Willie wasn't the only one eager to test Harold's mettle. Harold's own brother, Tostig, joined forces with Norway's king Harald Hardrada attacked from the north, meeting Harold II in the Battle of Stamford Bridge on 25 September. Harold II was victorious, but two days later, Duke William II set sail from Normandy, landing on the southern coast of England, at Pevensey, on September 28--the invasion was on.
William and his men headed north, Harold and his men headed south and the two armies met at 9am on October 14 in the legendary Battle of Hastings. By dusk, Harold II was dead as were an estimated 4,000 of his men, double the number lost by William.
William the Conqueror was crowned king of England on Christmas Day of that year and reigned until his death nearly 21 years later.