This apocryphal work includes two short letters. One letter claims to be written by the Syrian king Abgar (also known as Abgarus). The other claims to be a reply from Jesus.
According to early church historian Eusebius, he found these letters in the royal archives at Edessa and translated them from Syrian into Greek. He also included a story about the works of the apostle Thaddaeus.
In the legend, King Abgar was suffering from a serious disease. When he heard about the miracles of Jesus, he sent a messenger with a letter asking Jesus to come and heal him. He also offered Jesus a safe place to stay, away from the danger he faced in Jerusalem.
Jesus sends a letter in reply, in which he refuses the offer to take refuge but promises that after his ascension, he will grant Abgar's request by sending a disciple to him. (Later, the letter in the story was changed to an oral response in order to make the story agree with the tradition that Jesus left no writings.) Later, the apostle Thaddaeus went to Edessa, healed Abgar, and the royal household became followers of Christ.
A later version of the story appears in a writing called the Doctrina Addaei (written around AD 400). In that version, the reply from Jesus is spoken rather than written, and Abgar’s messenger brings back a painted portrait of Jesus instead of a letter. The portrait was displayed in the king’s palace.
The legend is similar to the one in the Greek Acts of Thaddaeus from the fifth or sixth century AD. However, in the Acts, Ananias (Abgar's messenger) returns with a face cloth that has a miraculous printing of Christ's face on it.
See also Apocrypha.