, which references Isbul, in the Archaeological Museum of
Philippi's
lapidarium.|alt=A damaged stone plate with a Medieval Greek inscription The office of the
kavhan was a hereditary title in the First Bulgarian Empire, monopolised by the members of the tentatively known "Kavhan family". In order to accede to that position, Isbul must have belonged to the Kavhan family, which is indirectly evidenced by his
Bulgar name. Historian Plamen Pavlov theorises that Isbul may have begun his career under the ruler
Krum (r. 803–814), and by the time of Krum's son Omurtag (r. 815–831), Isbul was already an influential noble. As he is referred to as a
kavhan and regent of the next ruler, Malamir (r. 831–836), it is conjectured that he had been appointed to the office at some point during Omurtag's rule. The earliest record of Isbul is the stone
epigraph known as the Malamir Chronicle, which states that Malamir "ruled together with
kavhan Isbul". The
Byzantines hoped to take advantage of Bulgaria's instability at the time, caused by the presence of the underage Malamir on the throne, and broke the long-lasting peace established with the
Byzantine–Bulgarian Treaty of 815, which they had initially reaffirmed upon Malamir's accession. In 836, As part of this campaign, Isbul and Malamir captured the
Thracian fortresses of Probaton (near
Adrianople) and
Bourzidon. After conquering these two fortresses, the Bulgarian troops reached
Philippopolis. As the defence forces of the city had fled, Isbul and Malamir entered negotiations with the population in order to persuade them to cede the fortress. Pavlov is of the opinion that the war ended with a continuation of the peace of 815. He believes that
Byzantine Emperor Theophilos (r. 829–842) was forced to make concessions to Bulgaria, which may have included the incorporation of Philippopolis and its close surroundings into the Bulgarian Empire. After the surprise death of Malamir in 836, the Bulgarian throne passed to Presian I (836–852), who too was likely underage. As evidenced by the
Presian Inscription from
Philippi, Isbul retained his position as
kavhan and his decisive influence in the Bulgarian court. In 837, the
Slavic tribe of the
Smolyani (
Smolenoi), who inhabited the lower
Nestos (Mesta) River and
Western Thrace near
Drama, rose against their Byzantine rulers. As the inscription is damaged, it is unclear who Presian and Isbul supported in that conflict. However, during this campaign the Bulgarian army conquered most of Macedonia including Philippi, where the inscription was found. From that point on, Isbul disappears from the sources, and there is no information about the date and circumstances of his death. ==Assessment and legacy==