The Theban Legion (also known as the Martyrs of Agaunum) figures in Christian hagiography as a Roman legion from Egypt —"six thousand six hundred and sixty-six men" — consisting of Christian soldiers who were martyred together in 286, according to the hagiographies of Maurice, the chief among the Legion's saints. Their feast day is held on September 22.

According to Eucherius of Lyon, c. 443–450, the Legion was garrisoned at Thebes in Egypt. The Legion was quartered in the East until the emperor Maximian ordered them to march to Gaul, to assist him against the rebels of Burgundy. The Theban Legion was commanded by Maurice (Mauritius), Candidus, Innocent, and Exuperius, all of whom are venerated as saints. At Saint-Maurice, Switzerland, then called Agaunum, the orders were given—since the Legion had refused to perform sacrifice according to the imperial cult —to decimate it by putting to death a tenth of its men. This act was repeated twice before the entire legion was put to death.

According to a letter written about 450 by Eucherius, Bishop of Lyon, bodies identified as the martyrs of Agaunum were discovered by Theodore (Theodulus), the first historically identified Bishop of Octudurum, who was present at the Council of Aquileia, 381 and died in 391. The basilica he built in their honor attracted the pilgrim trade; its remains can still be seen, part of the abbey begun in the early sixth century on land donated by King Sigismund of Burgundy.

The earliest surviving document describing "the holy Martyrs who have made Agaunum illustrious with their blood" is the letter of Eucherius, which describes the succession of witnesses from the martyrdom to his time, a span of about 150 years. The bishop had made the journey to Agaunum himself, and his report of his visit multiplied a thousandfold the standard formula of the martyrologies:

We often hear, do we not, a particular locality or city is held in high honour because of one single martyr who died there, and quite rightly, because in each case the saint gave his precious soul to the most high God. How much more should this sacred place, Agaunum, be reverenced, where so many thousands of martyrs have been slain, with the sword, for the sake of Christ.

As with many hagiographies, Eucherius' letter to Bishop Salvius reinforced an existing pilgrimage site. Many of the faithful were coming from diverse provinces of the empire, according to Eucherius, devoutly to honor these saints, and (important for the abbey of Agaunum) to offer presents of gold, silver and other things.

In the late 6th century, Gregory of Tours was convinced of the miraculous powers of the Theban Legion though he transferred the event to Cologne, where there was an early cult devoted to Maurice and the Theban Legion:

At Cologne there is a church in which the fifty men from the holy Theban Legion are said to have consummated their martyrdom for the name of Christ. And because the church, with its wonderful construction and mosaics, shines as if somehow gilded, the inhabitants prefer to call it the "Church of the Golden Saints". Once Eberigisilus, who was at the time bishop of Cologne, was racked with severe pains in half his head. He was then in a villa near a village. Eberigisilus sent his deacon to the church of the saints. Since there was said to be in the middle of the church a pit into which the saints were thrown together after their martyrdom, the deacon collected some dust there and brought it to the bishop. As soon as the dust touched Eberigisilus' head, immediately all pain was gone.