In 1386 Grand Duke of Lithuania Władysław II Jagiełło granted him possession of Ostroh castle and appointed him governor of Volhynia with the capital at Lutsk in 1387.
A folk song about this battle mentions Frederic, Ruthenian Prince of Ostrog, who left home and had assimilated Czech habits and language (Jan Długosz book XI, page 650).
[5] At Easter of the same year a group under the command of Frederic Prince of Ostrog, Jakub Nadobny from Rogów and Jan Kuropatwa from Łańcuchów Średniawa, attacked Jasna Góra Monastery, plundered it and destroyed Icon of Black Madonna of Częstochowa.
Then they were joined by Hussites from Lesser Poland and after an unsuccessful siege of Chojnice castle captured Tczew, took two thousand captives and besieged Gdańsk with no result.
He died as a monk and was buried at the Kyiv Pechersk Lavra under the name Teodosii (Theodosius) and was later canonized (his feast day is 24 August).