The mandylion was a semi-legendary relic of Christianity, a square or rectangular piece of cloth bearing the image of the face of Jesus Christ, and first appearing in the city of Edessa in the 6th century.
According to Eusebius, King Abgar of Edessa was afflicted of an illness, and hearing of the miracles of Jesus as a healer he sent a letter to Him, asking if He would come to his aid. Jesus responded that He could not come, but would send a disciple in His place, which He does. Thaddeus comes in Jesus' place and heals him ; according to variants of this story King Abgar is left with the cloth image of Jesus, beginning with the Doctrine of Addai (ca. 400 A.D.) in which a court painter created an image of the Lord and "brought with him to Abgar the king, his master. And when Abgar the king saw the likeness, he received it with great joy, and placed it with great honor in one of his palatial houses." 
The mandylion would surface again around 525 when Edessa was flooded by the Daisan River. Workmen repairing one of the city's gates discovered a niche with the cloth inside; the mandylion was declared to be Acheiropoietos (Greek: Αχειροποίητος), "not made by hands", meaning that it was a miraculous image created supernaturally and not by man. The mandylion stayed in Edessa as a means of protection for the city from harm until taken to Constantinople in 944, where it was received with great fanfare by Emperor Romanus I. There it stayed until disappearing in the sack of the city during the Fourth Crusade in 1204.
Shroud of Turin?
Author Ian Wilson wrote in his history of the Shroud of Turin that the mandylion may have been the shroud itself, "doubled in four" with the face of the shroud placed prominently, and set inside a latticework frame that showed only the face. Wilson also drew upon several accounts from Constantinople, such as the Codex Vossianus Latinus (Q 69) and a 1207 letter from Nicholas Mesarites, in which the mandylion was described as being a full-body burial cloth, and not just the face alone. Wilson further elaborated that when the mandylion disappeared, it found its way to France under one Geoffroi de Charny, one of the Knights Templar, who was the first confirmed owner of the shroud.