857

By place

Byzantine Empire

• Emperor Michael III, under the influence of his uncle Bardas, banishes his mother Theodora to the Gastria Monastery. Bardas, the de facto regent, becomes the most powerful person in the Byzantine Empire.

  Europe

• November - Erispoe, ruler of Brittany, is assassinated by his cousin Salomon and followers, in the church at Talensac. King Charles the Bald acknowledges Salomon as the rightful 'king' of Brittany.
• A Danish Viking fleet raids the cities of Dorestad, Paris and Orléans. Others sail up the Oise River, ravaging Beauvais and the abbey of Saint-Germer-de-Fly.
• Viking chieftain Rorik, with the agreement of King Lothair II, leaves Dorestad with a fleet and forces his rival Horik II to recognise him as ruler over Denmark.

  By topic

Medicine

• The first recorded major outbreak of ergotism kills thousands of people in the Rhine Valley. They have eaten bread made from rye infected with the ergot fungus parasite Claviceps purpurea.