The legend of the ten thousand martyrs is believed to have originated sometime in the Middle Ages and had no factual historical backing. The legend tells of a Roman army, led by emperors Adrian and Anoninus, which attempted to suppress a revolt by the people of the Gadara region in the Middle-East. The ranks of the revolution were bolstered by people from a simultaneous revolt in the Euphrates region, and their combined forces managed to overwhelm the Romans.

As the rest of the army fled, nine thousand soldiers were stopped by the voice of an angel, telling them to stand strong despite the odds stacked against them. Turning to face their enemy, they managed to completely defeat and rout the revolutionaries. After this, they were taken by the angel to Mount Ararat, where they were instructed in the teachings of Christ and the faith.

Meanwhile, the emperors Adrian and Anoninus, hearing of the battle, sent messengers to the nine thousand inviting them to return so as they may celebrate and offer sacrifices to the Roman gods as thanks for their victory. However, the converted Romans refused, citing their newfound faith, and were thus branded as heretics and rebels.

The emperors then enlisted the help of several local vassals to help put down the nine thousand and marched on Mount Ararat, where they demanded that the converts denounce their faith. Upon their refusal, the emperors ordered their archers and slingers to attack, but the missiles rebounded harmlessly. Seeing this miracle, a thousand more soldiers converted to the faith. At this point the emperors ordered the ten thousand martyrs to be crucified.
See: Agnolo Bronzino. Martyrdom of the Ten Thousand Martyrs.
Vittore Carpaccio. The Ten Thousand Crucifixions of Mount Ararat.
Albrecht Durer. The Martyrdom of the Ten Thousand.