Before the construction of the first monastic establishment in the 8th century, the island was called Mont Tombe (Latin: tumba). According to legend, the Archangel Michael appeared in 708 to St. Aubert, the bishop of Avranches, and instructed him to build a church on the rocky islet. Aubert repeatedly ignored the angel's instruction until Michael burned a hole in the bishop's skull with his finger.[11]
Unable to defend his kingdom against the assaults of the Vikings, the king of the Franks agreed to grant the Cotentin peninsula and the Avranchin, including Mont-Saint-Michel, to the Bretons in the 867 Treaty of Compiègne. This marked the beginning of the brief period of Breton possession of the Mont. In fact, these lands and Mont Saint-Michel were never really included in the duchy of Brittany and remained independent bishoprics from the newly created Breton archbishopric of Dol. When Rollo confirmed Franco as archbishop of Rouen, these traditional dependences of the Rouen archbishopric were retained in it.
The mount gained strategic significance again in 933 when William "Long Sword" annexed the Cotentin Peninsula from the weakened Dukes of Brittany. This made the mount definitively part of Normandy, and is depicted in the Bayeux Tapestry, which commemorates the 1066 Norman conquest of England. Harold, Earl of Wessex is pictured on the tapestry rescuing two Norman knights from the quicksand in the tidal flats during a battle with Conan II, Duke of Brittany. Norman ducal patronage financed the spectacular Norman architecture of the abbey in subsequent centuries.