A great-grandson of prince Vlastimir, Caslav was born and raised in the Bulgarian capital Preslav, at the court of tsar Symeon. Sharing the fate of numerous medieval princes and throne pretenders, his grandfather was exiled there following dynastic struggles upon Vlastimir's death, and his father, in turn, made a fatal bid for the Serbian throne in the 890s.

Meanwhile, the early 10th century saw Serbia as a player forced into the larger Byzantine-Bulgarian conflict, precipitated by the aggressive moves of the powerful Symeon. Generally, at this juncture, Serbs tended to prefer the more civilized, benevolent, and distant Constantinopolitan state and its diplomacy, but the Bulgarian aggressive proximity and pressure often demanded equal or greater concessions and occasional military action. Nonetheless, the Serbian princes played this game with some success, until finally, Symeon decided to eliminate this side threat to his perennial quest for the universal Byzantine throne.

A large force was sent into Serbia around the year 924, ostensibly to install Caslav as the new grand Zupan acceptable to Bulgaria. Some zupans fled, yet others were summoned to pay homage to the pretender to the throne. The latter proved the gullible ones, as Bulgarians treacherously took them all, Caslav included, back as prisoners, thus leaving Serbia as an annexed province until the tsar's death in 927.

Soon after this, under conditions of declining Bulgar might, Caslav reemerges, having escaped from Preslav - now in the more honorable role of liberator of his dominion. Serbia had in the meantime been severely ravaged and depopulated, with many fleeing to Greece and Croatia. Caslav then struck a deal with Byzantine emperor Romanus I, recognizing his supreme authority in exchange for economic help. This allowed his realm to be rebuilt, attract back the emigrants, and apparently thrive in relative prosperity during his long reign.

Caslav's state, the main Serbian principality of its day, was larger than that of Vlastimir and encompassed areas of present-day western Serbia, eastern Bosnia, and eastern Hercegovina. During this rule, a trend of tribal unification can be noted, probably owing to the Bulgar menace, as well as to Christian conversion. And while the Church schism was formally another hundred years in the future, Caslav's leaning towards Constantinople for spiritual apart from political support - with its orthodoxy and right to a comprehensible Slavonic liturgy, as opposed to the rival Roman option - was to have important consequences in the centuries hence.

Caslav died around 960 in the battle against Hungarian raiders. Having arrived from the east and partaken in the Byzantine-Bulgarian wars, these people now settled in the Pannonian plane north of Serbia and entered forever the political picture of the northern Balkans. With Caslav's death, Serbia again splits into smaller units, and there also comes an end to the house of Viseslav, which ruled central Serbian lands possibly for three hundred years - the last hundred or so of which we have some specific information on.