The Doctrina Addaei is an expanded Syriac version of the Letters of Christ and Abgarus, written around AD 400. It tells the story of the meeting between Abgar, the king of Edessa, and Jesus Christ.
King Abgar was suffering from an unknown illness. He sent a messenger to Jesus with a written request for healing and an offer of safety in his kingdom. Jesus sent back an oral reply, promising to send one of his followers to Abgar after ascending to heaven. The messenger of the king brought back with him a self-portrait of Jesus, which became a treasured object in Edessa.
Later, Addai (sometimes called Thaddeus) visited the kingdom. He healed Abgar and established Christianity there.