My POD would be 1025, when Junior king Henry I of the Franks, along with his mother, Constance of Arles and younger brother, Prince Robert, revolted against his father, Senior King Robert II, the revolt is a success, seeing, King Robert II, killed in the battle of Melun.
However, instead of going to plan, Junior king Henry I, is usurped when his mother, Constance of Arles, supported his sibling, Robert as heir to the old king, Henry is killed by Robert.
William is born in 1028, the son of the unmarried Robert I, Duke of Normandy, by Robert's mistress Herleva. His illegitimate status, caused some difficulties for William and his father, in Normandy, when the king found out that the old king Robert II and his junior king, Henry I, Robert I, Duke of Normandy, saw this as his opportunity to gain favour, with the 17 year old, King Robert III.
Robert I, Duke of Normandy, travels to the capital, but his movement is assumed to be a counter-revolt, the young king, scared of being punished for killing his father and brother, commits suicide.
Robert, now finds the throne and crown, on offer and takes the chance to be crowned, King Robert IV of the Franks, naming his son as a legitimate heir.
When Robert died in 1055, his son and heir, William was crowned King William I of the Franks, with his power fully established and his hold on his throne secure, he was able to launch the Norman conquest of England in 1066.
However the rest of his life was marked by struggles to consolidate his hold over England and his continental lands, a struggle that would come apart at his death in 9 September 1087, when his eldest son, Robert Curthose, proclaimed himself King Robert V of the Franks, while his third son, claimed the English throne as King William II.