A precious source for us when studying the Slavs in the Balkans at the end of the 6th century is the “Miracles of St. Demetrius”. This source was written in Greek and originates from the 7th century. There are two parts(two books). The first book was written around the year 620 and it is considered that its author was the Archbishop of Thessaloniki, Jovan (John) ; the author of the second book remains unknown and is believed to have been composed around 680. The content of this source refers to the history of the city of Thessaloniki through the late sixth and most of the seventh century, and an important factor in the preservation of the city its patron – Saint Demetrius.
According to the source, in 584, a group of five thousand Slavs attacked Thessaloniki. The attack of the Slavs, “beasts in human form” , takes place in October when the citizens were celebrating their patron saint’s day. It is mentioned that there were relatively few Slavs, but that they were very strong. Here the question arises whether the citizens of Thessaloniki were familiar with a smaller but selected group of Slavs? It is interesting to note that in that chaos and flames, the citizens of Thessaloniki recognized the Slavic cries. This only speaks to the fact that this is not the first meeting of citizens with Slavs from the Thessaloniki area. This attack was repulsed, but it was only a matter of time before a new one was organized.
It turned out that the attack was planned very quickly, because already in 586 the united Slavs with the Avars attacked Thessaloniki “the richest, most respected and most Christian city of Thrace and Illyricum”. In this episode, the saint appears as a warrior, together with his holy warriors. According to the story, the terrified citizens of Thessaloniki saw angel warriors on their walls who defended the city at a crucial moment, and they were commanded by a fiery man on a horse whose white cloak stood out in the crowd. Again the miracle,Demetrius, defended his city. However, this source also shows the other side of the siege – shortages and famine caused by the siege. Of course, the patron showed his power once again and the grain ship sailed to Thessaloniki and saved the people from death.