Rostyslav Mykhailovych [Myxajlovyč], b and d ? Kyivan Rus’ prince; son of Mykhail Vsevolodovych of Chernihiv. Playing a supporting role for his father's ambitions, he was briefly prince of Novgorod the Great (1229–30) and was later seated in princely Halych (ca 1239) and Chernihiv (1241). Danylo Romanovych retook Halych, and a protracted struggle ensued for lands in the Principality of Galicia-Volhynia, until Danylo defeated the army formed through an alliance of Rostyslav, his father-in-law, Béla IV of Hungary, and his brother-in-law, Bolesław the Bashful of Little Poland, in the Battle of Yaroslav (1245). Rostyslav fled to Hungary, where he headed a small principality in Slavonia.
Encyclopedia of Ukraine