Aleksii Slav was a nephew of Ivan Asen I of Bulgaria, Peter IV of Bulgaria and Kaloyan of Bulgaria. Henri de Valenciennes records Aleksii was a first cousin of Boril of Bulgaria. According to The Late Medieval Balkans (1987) by John V.A. Fine, Jr., Aleksii opposed the succession of Boril to the throne of the Second Bulgarian Empire in 1207. He seceded and established his own state in the Pirin region with the town of Melnik as its capital. Henri de Valenciennes records that Aleksii sought an alliance with Henry of Flanders, Latin Emperor. The alliance was sealed with his marriage to an illegitimate daughter of Henry in 1208. Aleksii received the title of despot from his father-in-law. According to Fine, the second marriage of Aleksii to a niece of Maria took place c. 1216. With Henry deceased, Aleksii was seeking an alliance with Michael