The massacre of the Theban legion is said to have taken place under Diocletian between 285 and 306 at Agaune (today Saint-Maurice) in Valais.
Cited by a single source, of religious origin, from the 5th century, this episode is sometimes considered legendary, in particular because of the non-existence of this legion in the list of Roman legions of the time. In any case, it seems extremely unlikely that it was an entire legion that perished:either its leaders were killed, or it was a smaller troop.
According to this tradition, passing through Agaune, Saint Maurice, commander of this Theban legion, refused to sacrifice to the cult of the emperor. He was put to death, along with his companions.
His remains were exhumed by Theodore, the first known bishop of Octodure, who founded a sanctuary in Agaune, which took the name of Saint-Maurice.
This sanctuary became an abbey, the Abbey of Saint-Maurice d'Agaune, in 515 under the reign of the Burgundian king Sigismond. The first king of Burgundy transjurane, the count of Auxerre Rodolphe, was crowned there in 888.
It is difficult to know if a real historical event is the basis of this legend. The reign of Diocletian is a period frequently attributed to legendary martyrs, and is therefore not a reliable date. Some researchers have put forward the hypothesis of a connection between it and the battle which seems to have taken place around 275-277 when the Alamans, after having raided the Swiss Plateau, seem to have been stopped at the cluse of Saint-Maurice, defile easily defensible. An inscription speaking of the death of Junius Marinus during combat has been found.
Saint Victor, an officer in the Theban legion, who had escaped the Agaune massacre and had taken refuge in Marseilles, suffered martyrdom - crushed under a baker's millstone - in this city on July 21, 303 (or 304 according to d other sources) for refusing to recant his Christian faith.