He was first mentioned as one of the nobles disputing Tsar Boril's ascension to the Bulgarian throne. He married the daughter of Latin Emperor Henry of Flanders in November 1208, leaving Veliko Tarnovo and establishing himself as an independent ruler over the most part of the Rhodope Mountains. Alexius Slav became a vassal of Henry after the Bulgarian defeat near Plovdiv. Henry promised to support his aspirations for the Bulgarian throne and awarded Alexius Slav the title of despot (despotes).