The progenitor of the noble house of Bulat was a knight and nobleman called Bulat Kustražić († 1440). He is mentioned only in one historical document, where the Vlach noblemen are listed by prince Ivan VI Frankopan († 1436) on 18th March 1436 at the castle Klis. In this document the Vlachs receive a confirmation of their special privileges, such as the ability to elect their own prince, duke and to have their own courts where they could not be tried by neither Croats nor Serbs. The property Bulat Kustražić owned was called Kraljske Mirine and was located in Biskupija near Knin, Croatia. Many things are not certain about the life of him, but we know Bulat Kustražić died in battle during the conquest of Omiš and Poljice in service to grand duke Stjepan Vukčić Kosača († 1466) and is buried at the necropolis in Trilj. He received his name from a type of steel called bulat which was used by medieval knights for swords.