William the Conqueror
Sometimes known as William the Bastard having been born the illegitimate son of the Duke of Normandy. He inherited the Duchy upon his father's death, but it is for succesfully leading the Norman conquest of England that he is best known.
William first attempted to gain England through diplomatic manouveuring. He first claimed that Edward the Confessor had named him successor in 1051, and then in 1064 he somehow managed to get Harold Godwinson to swear fealty to him and relinquish his claim to the throne. The latter event almost certainly took place under duress and the former may not have happened at all, but he used both as a pretext for invasion when the witan declared Harold king in January 1066.
William delayed his assault until late September waiting for favourable winds. All this stretched Harold's resources as not only did he need to stay on high military alert for several months, but he also had to fight at the demanding Battle of Stamford Bridge.
William's victory at the Battle of Hastings marked a turning point in English history. Not only was it the most succesful invasion of Britain by a foreign force, it also marked the begining of a new dynasty. All kings of England number themselves from the ascension of the Norman kings.
Having been crowned in 1066, William set about securing his new kingdom. Uprisings were quelled brutally, Norman lords were installed into places of power and by 1072 the whole of England had been united once more.