The most consequential part of the Abgar story is the part Eusebius did not tell. Around the year 400 a Syriac narrative called the Doctrine of Addai retells the legend and adds a detail: the king sends a court painter to Judaea, and the painter returns with a portrait of Jesus painted from life, which is set up in the palace. By the sixth century the portrait has become something else again, an image not made by human hands, imprinted on cloth when Jesus pressed it to his face. Under that description it acquires a name, the Mandylion, and a role in the defence of icons.
The object mattered politically as well as devotionally. Edessa was said to be protected by it, and the promise in the letter that no enemy would master the city was read as a guarantee attached to the relic. In 944 a Byzantine army besieging Edessa accepted the image as the price of withdrawal, and it was carried in procession into Constantinople on 15 August, an event still commemorated in the Byzantine calendar. It disappeared from the record after the sack of Constantinople in 1204.
For the history of Christian art the sequence matters more than the object. In three stages, a letter with no image becomes a painted portrait, and a painted portrait becomes an image made without hands. That progression supplied the eastern church with its founding case for the legitimacy of icons: if the prototype was made by Christ himself and not by a painter, then no charge of idolatry can reach it. Several later relics and traditions, including the western Veronica, follow the same logic.