The Unknown Archon was the first Serbian ruler. He led the Serbs into the Balkans during the reign of Byzantine Emperor Heraclius (610–641), as mentioned in Emperor Constantine VII’s De Administrando Imperio.
De Administrando Imperio
The Serbs are descended from the unbaptized Serbs, also called ‘white’, who live beyond Turkey in a place called by them Boiki, where their neighbour is Francia, as is also Great Croatia, the unbaptized, also called ‘white’: in this place, then, these Serbs also originally dwelt. But when two brothers succeeded their father in the rule of Serbia, one of them, taking a moiety of the folk, claimed the protection of Heraclius, the emperor of the Romans, and the same emperor Heraclius received him and gave him a place in the province of Thessalonica to settle in, namely Serbia, which from that time has acquired this denomination.
Now, after some time these same Serbs decided to depart to their own homes, and the emperor sent them off. But when they had crossed the river Danube, they changed their minds and sent a request to the emperor Heraclius, through the military governor then governing Belgrade, that he would grant them other land to settle in.
And since what is now Serbia and Pagania and the so-called country of the Zachlumi and Trebounia and the country of the Kanalites were under the dominion of the emperor of the Romans, and since these countries had been made desolate by the Avars (for they had expelled from those parts the Romans who now live in Dalmatia and Dyrrachium), therefore the emperor settled these same Serbs in these countries, and they were subject to the emperor of the Romans; and the emperor brought elders from Rome and baptized them and taught them fairly to perform the works of piety and expounded to them the faith of the Christians.
And since Bulgaria was beneath the dominion of the Romans … when, therefore, that same Serbian prince died who had claimed the emperor’s protection, his son ruled in succession, and thereafter his grandson, and in like manner the succeeding princes from his family.”