The reign of William the Conqueror was marked by uprisings, noble conspiracies, and threats from within his own family. These 15 plots reveal how fragile power could be for a Norman duke—and later, King of England.
By Lorris Chevalier
William the Conqueror spent much of his life not merely conquering lands but surviving betrayal. The illegitimate son of Robert I of Normandy and Herleva of Falaise, he inherited the Duchy of Normandy as a child and the English crown as an adult. Yet from his earliest years until his death in 1087, treachery, often from his own kin, shadowed his rule.
William’s life was shaped as much by the need to survive as by his ambition to conquer. An illegitimate child who became duke at a young age, he faced assassination plots and noble conspiracies long before he reached adulthood. Even after securing the English crown, he continued to confront rebellion, from former allies, rival kings, Anglo-Saxon claimants, powerful earls, and ultimately his own son.
Some of his most formidable enemies were members of his own family: cousins, uncles, and even his designated heir. In the eleventh century, dynastic politics made kinship both the foundation of authority and a persistent source of danger.
Although William died in 1087 from injuries sustained during a military campaign in France rather than through treachery, his reign reveals a fundamental reality of medieval rulership: betrayal was not an exception, but an enduring part of power.
1. The Anarchy of His Minority (1035–1040)
An illustration in the late 11th-century Ramsey Benedictional (Paris, Bibliothèque nationale de France, ms. latin 987, f. 111r) showing the second coronation of William the Conqueror in 1070.
When William became Duke of Normandy in 1035 at around eight years old, Normandy descended into chaos. With no adult ruler, rival Norman barons (many distantly related to the ducal house) sought to control or eliminate the young duke.
Several of his guardians were assassinated in rapid succession. The instability was not merely political disorder; it was a calculated struggle for power over a vulnerable child whose legitimacy was already questioned due to his birth.
2. The Death of Alan III of Brittany (1040)
Alan III of Brittany, William’s cousin (his aunt Hawise of Normandy was Alan’s mother), had assumed the role of protector. Yet Alan also had a blood claim to Normandy.
He died suddenly (possibly poisoned) while campaigning in Normandy. Though the precise circumstances remain unclear, his ambitions and family connection made his guardianship politically dangerous. His death removed one potential usurper.
3. The Assassination of Gilbert of Brionne (1040)
Gilbert of Brionne, another of William’s protectors and a powerful noble with ducal blood, was murdered at the instigation of rival barons.
His death further exposed the young duke. It demonstrated that proximity to William was deadly and that factions were willing to kill those closest to him in order to destabilise ducal authority.
4. The Murder of Osbern de Crépon in William’s Chamber (c. 1040–1041)
One of the few representations of William “the Conqueror” from Jumièges Abbey. The crown ring refers to his kingship. Photo by Xbra / Wikimedia Commons
Osbern de Crépon, William’s seneschal and close relative by alliance, was killed in the very chamber where the boy duke slept.
The assassin was reportedly the son of Roger I of Montgomery, a shocking act that underscored how even noble families entrusted with ducal service could harbour violent ambition. William himself may have witnessed the murder.
5. The Conspiracy of 1046 – Attempted Assassination at Valognes
In 1046 a coalition of western Norman lords conspired to murder William and replace him with his cousin Guy of Brionne, son of Renaud of Burgundy and Adelaide, daughter of Richard II of Normandy. This made Guy a direct cousin of William and a serious dynastic rival.
Warned at the last moment, William fled by night from Valognes. The assassination attempt nearly succeeded. The rebellion culminated in the Battle of Val-ès-Dunes (1047) where, with the support of King Henry I of France, William crushed his rebellious kinsmen.
6. The Revolt of William of Arques (1053–1054)
William of Arques, William’s own uncle, rebelled and sought assistance from the King of France.
This was no distant baronial dispute but a family rebellion aimed at displacing him. After a siege, the uncle was exiled and his lands confiscated.
7. Royal Betrayal by Henry I of France (1052–1057)
Henry I of France depicted in a 14th-century manuscript
Henry I of France had once supported William at Val-ès-Dunes. Yet as William’s power grew, Henry reversed alliances and repeatedly invaded Normandy.
Though not a familial betrayal, this shift illustrates how even former allies could turn against the Norman duke when his strength threatened their own.
8. The Poisoning of Conan II of Brittany (1066)
Conan II of Brittany died suddenly in 1066 during conflict with William. Contemporary rumours suggested poisoning.
Although this event reflects William’s own ruthless politics, it also shows the volatile and treacherous atmosphere in which he operated, where poisoning was a recognised political tool.
9. Harold Godwinson’s “Perjury” (1066)
Harold Godwinson swearing oaths to William in the Bayeux Tapestry
Harold Godwinson had allegedly sworn to support William’s claim to the English throne.
When Harold accepted the crown in January 1066 following the death of Edward the Confessor, William viewed this as personal betrayal and perjury. Though not a blood relative, Harold’s oath (if genuine) formed the moral justification for William’s invasion.
10. The Northern Rebellions and Danish Invasion (1069–1070)
After William’s coronation, English nobles rallied behind Edgar Ætheling, the last male of the old royal house.
Danish forces under King Sweyn II supported rebellion. While not assassination attempts, these uprisings aimed to remove William from the throne entirely. The resulting “Harrying of the North” was William’s brutal response to repeated betrayal.
11. The Revolt of Ralph de Gaël and Roger de Breteuil (1075)
Waltheof’s confession of his involvement in a plot against King William I – drawing by Gordon Frederick Browne in 1909 – Wikimedia Commons
The so-called Revolt of the Earls was led by Ralph de Gaël and Roger de Breteuil.
They were joined, perhaps reluctantly, by Waltheof, who had married William’s niece Judith. This was a betrayal through a marriage alliance, and Waltheof’s execution in 1076 marked the last execution of an Anglo-Saxon earl.
12. The First Rebellion of Robert Curthose (1077–1079)
The most painful betrayal came from William’s eldest son, Robert Curthose.
Frustrated at being denied real authority, Robert fled and allied himself with William’s enemies, including King Philip I of France. At the Siege of Gerberoy (1079), Robert reportedly unhorsed and wounded his own father in combat. This was not merely rebellion: it was filial war.
13. Philip I of France’s Interference (Late 1070s)
Philip I depicted in the Grandes Chroniques de France . Bibliothèque Sainte-Geneviève,Ms. 782
Philip I of France repeatedly encouraged Robert Curthose and other rebels.
Philip’s support transformed a domestic family dispute into international conflict, compounding the personal betrayal with diplomatic hostility.
14. The Second Rebellion of Robert Curthose (1083–1086)
After a brief reconciliation, Robert again turned against his father. The conflict deepened following the death of Queen Matilda in 1083, who had often mediated between father and son.
The struggle revealed a dynastic fracture at the heart of William’s legacy.
15. The Arrest of Odo of Bayeux (1082)
Perhaps the most shocking internal betrayal involved William’s own half-brother, Odo of Bayeux, son of Herleva by Herluin of Conteville.
Odo, Bishop of Bayeux and Earl of Kent, was one of William’s closest companions during the Conquest. Yet he was arrested and imprisoned, allegedly for plotting an Italian campaign and overreaching in power. Though not an assassination attempt, it was a profound breach of familial trust.
Dr Lorris Chevalier, who has a Ph.D. in medieval literature, is a historical advisor for movies, including The Last Duel and Napoleon. Click here to view his website.
The reign of William the Conqueror was marked by uprisings, noble conspiracies, and threats from within his own family. These 15 plots reveal how fragile power could be for a Norman duke—and later, King of England.
By Lorris Chevalier
William the Conqueror spent much of his life not merely conquering lands but surviving betrayal. The illegitimate son of Robert I of Normandy and Herleva of Falaise, he inherited the Duchy of Normandy as a child and the English crown as an adult. Yet from his earliest years until his death in 1087, treachery, often from his own kin, shadowed his rule.
William’s life was shaped as much by the need to survive as by his ambition to conquer. An illegitimate child who became duke at a young age, he faced assassination plots and noble conspiracies long before he reached adulthood. Even after securing the English crown, he continued to confront rebellion, from former allies, rival kings, Anglo-Saxon claimants, powerful earls, and ultimately his own son.
Some of his most formidable enemies were members of his own family: cousins, uncles, and even his designated heir. In the eleventh century, dynastic politics made kinship both the foundation of authority and a persistent source of danger.
Although William died in 1087 from injuries sustained during a military campaign in France rather than through treachery, his reign reveals a fundamental reality of medieval rulership: betrayal was not an exception, but an enduring part of power.
1. The Anarchy of His Minority (1035–1040)
When William became Duke of Normandy in 1035 at around eight years old, Normandy descended into chaos. With no adult ruler, rival Norman barons (many distantly related to the ducal house) sought to control or eliminate the young duke.
Several of his guardians were assassinated in rapid succession. The instability was not merely political disorder; it was a calculated struggle for power over a vulnerable child whose legitimacy was already questioned due to his birth.
2. The Death of Alan III of Brittany (1040)
Alan III of Brittany, William’s cousin (his aunt Hawise of Normandy was Alan’s mother), had assumed the role of protector. Yet Alan also had a blood claim to Normandy.
He died suddenly (possibly poisoned) while campaigning in Normandy. Though the precise circumstances remain unclear, his ambitions and family connection made his guardianship politically dangerous. His death removed one potential usurper.
3. The Assassination of Gilbert of Brionne (1040)
Gilbert of Brionne, another of William’s protectors and a powerful noble with ducal blood, was murdered at the instigation of rival barons.
His death further exposed the young duke. It demonstrated that proximity to William was deadly and that factions were willing to kill those closest to him in order to destabilise ducal authority.
4. The Murder of Osbern de Crépon in William’s Chamber (c. 1040–1041)
Osbern de Crépon, William’s seneschal and close relative by alliance, was killed in the very chamber where the boy duke slept.
The assassin was reportedly the son of Roger I of Montgomery, a shocking act that underscored how even noble families entrusted with ducal service could harbour violent ambition. William himself may have witnessed the murder.
5. The Conspiracy of 1046 – Attempted Assassination at Valognes
In 1046 a coalition of western Norman lords conspired to murder William and replace him with his cousin Guy of Brionne, son of Renaud of Burgundy and Adelaide, daughter of Richard II of Normandy. This made Guy a direct cousin of William and a serious dynastic rival.
Warned at the last moment, William fled by night from Valognes. The assassination attempt nearly succeeded. The rebellion culminated in the Battle of Val-ès-Dunes (1047) where, with the support of King Henry I of France, William crushed his rebellious kinsmen.
6. The Revolt of William of Arques (1053–1054)
William of Arques, William’s own uncle, rebelled and sought assistance from the King of France.
This was no distant baronial dispute but a family rebellion aimed at displacing him. After a siege, the uncle was exiled and his lands confiscated.
7. Royal Betrayal by Henry I of France (1052–1057)
Henry I of France had once supported William at Val-ès-Dunes. Yet as William’s power grew, Henry reversed alliances and repeatedly invaded Normandy.
Though not a familial betrayal, this shift illustrates how even former allies could turn against the Norman duke when his strength threatened their own.
8. The Poisoning of Conan II of Brittany (1066)
Conan II of Brittany died suddenly in 1066 during conflict with William. Contemporary rumours suggested poisoning.
Although this event reflects William’s own ruthless politics, it also shows the volatile and treacherous atmosphere in which he operated, where poisoning was a recognised political tool.
9. Harold Godwinson’s “Perjury” (1066)
Harold Godwinson had allegedly sworn to support William’s claim to the English throne.
When Harold accepted the crown in January 1066 following the death of Edward the Confessor, William viewed this as personal betrayal and perjury. Though not a blood relative, Harold’s oath (if genuine) formed the moral justification for William’s invasion.
10. The Northern Rebellions and Danish Invasion (1069–1070)
After William’s coronation, English nobles rallied behind Edgar Ætheling, the last male of the old royal house.
Danish forces under King Sweyn II supported rebellion. While not assassination attempts, these uprisings aimed to remove William from the throne entirely. The resulting “Harrying of the North” was William’s brutal response to repeated betrayal.
11. The Revolt of Ralph de Gaël and Roger de Breteuil (1075)
The so-called Revolt of the Earls was led by Ralph de Gaël and Roger de Breteuil.
They were joined, perhaps reluctantly, by Waltheof, who had married William’s niece Judith. This was a betrayal through a marriage alliance, and Waltheof’s execution in 1076 marked the last execution of an Anglo-Saxon earl.
12. The First Rebellion of Robert Curthose (1077–1079)
The most painful betrayal came from William’s eldest son, Robert Curthose.
Frustrated at being denied real authority, Robert fled and allied himself with William’s enemies, including King Philip I of France. At the Siege of Gerberoy (1079), Robert reportedly unhorsed and wounded his own father in combat. This was not merely rebellion: it was filial war.
13. Philip I of France’s Interference (Late 1070s)
Philip I of France repeatedly encouraged Robert Curthose and other rebels.
Philip’s support transformed a domestic family dispute into international conflict, compounding the personal betrayal with diplomatic hostility.
14. The Second Rebellion of Robert Curthose (1083–1086)
After a brief reconciliation, Robert again turned against his father. The conflict deepened following the death of Queen Matilda in 1083, who had often mediated between father and son.
The struggle revealed a dynastic fracture at the heart of William’s legacy.
15. The Arrest of Odo of Bayeux (1082)
Perhaps the most shocking internal betrayal involved William’s own half-brother, Odo of Bayeux, son of Herleva by Herluin of Conteville.
Odo, Bishop of Bayeux and Earl of Kent, was one of William’s closest companions during the Conquest. Yet he was arrested and imprisoned, allegedly for plotting an Italian campaign and overreaching in power. Though not an assassination attempt, it was a profound breach of familial trust.
Dr Lorris Chevalier, who has a Ph.D. in medieval literature, is a historical advisor for movies, including The Last Duel and Napoleon. Click here to view his website.
Click here to read more from Lorris Chevalier
Further Readings:
David Bates, William the Conqueror (George Philip, 1989)
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