The Slavs that migrated soon formed their own realm, the Principality of Littoral Croatia, ruled by Slavic princes. A separate tribe called Guduscani lived in the northwestern part of Roman Dalmatia. Borna of Croatia (803–821) was one of the earliest recorded rulers of the Principality of Littoral Croatia. In 806 the Principality of Dalmatia was temporarily added to the Frankish Empire, but the cities were restored to Byzantium by the Treaty of Aachen in 812. The treaty had also slightly expanded the Principality of Dalmatia eastwards.