On 28 September 995, Boleslaus and his confederate Vršovci stormed Libice in southern Bohemia and massacred the members of the Slavník dynasty.
He was the eldest child and only son of Bolesław II, Duke of Cieszyn, by his wife Anna, daughter of Ivan Vladimirovich, Prince of Bielsk.
While Siemowit's son Duke Konrad II (1264–1294) moved his residence to Czersk he and his brother Boleslaus II entered into a long-term conflict over the Polish seniorate with their Kuyavian relatives and the Silesian Piasts, which estranged them from the Piast monarchy.