King of the Slavs (Latin: rex Sclavorum, Sclavorum rex) was a title denoting some Slavic rulers, as well as Germanic rulers that conquered Slavs, in the Middle Ages in European sources, such as Papal correspondence.
Papal use is bolded.
Addressed (in 1288) by Pope Nicholas IV as 'Rex Sclavorum'; he signed himself (in 1305 or 1307) in a document still extant in the Venetian archives as ' Stephanus Urosch Rex Serbicarum terrarum et maritimarum '. a [Stefano Urosio.] ...