Legends associate the construction of the monastery with the name of Charlemagne, who was forced to make a stop here on his way because his army was stricken with illness (presumably the plague). According to legend, an angel appeared to the ruler in a dream and told him how to overcome the illness, pointing to a medicinal herb growing in the area.

But there is a more realistic version of the origin of the abbey:

Construction of a Benedictine monastery on this site began in 770 for pilgrims traveling from the north to Rome. On December 29, 814, Louis the Pious signed a decree essentially granting the abbey imperial patronage.