Isbul () (fl. 820s–830s) was the kavhan, or first minister, of the First Bulgarian Empire during the reigns of Omurtag, Malamir and Presian I. Appointed to the kavhan office under Omurtag, Isbul was a regent or co-ruler of the underage Malamir and his successor Presian. Under Malamir and Presian, Isbul headed Bulgaria's successful campaigns against the Byzantines in southern Thrace and Macedonia, which led to a significant territorial expansion of the Bulgarian realm. As a co-ruler of Malamir, Isbul also financed the construction of a water conduit in the capital Pliska. As second-in-command, Isbul held enormous power and wealth, and was unusually often mentioned beside the name of the ruler in inscriptions. Due to his merits, Isbul has been described as an architect of medieval Bulgarian statehood by historians. Biography 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". Malamir was the youngest son of Omurtag and must have been considered too young to rule by himself, so that a regent had to be appointed. Omurtag specified Malamir as his heir because his eldest son, Enravota, was a Christian. 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, Isbul was in charge of the Bulgarian forces which repulsed the Byzantine invasion and proceeded to raid into Byzantine territory. 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.