Boleslav II the Pious (died February 7 999 ) was the son of Boleslav I the Cruel. Boleslav II became Duke (or prince) of Bohemia in 972. Boleslav maintained good relations with the German Kingdom, and in 975 supported Otto II in his civil war against Henry of Bavaria. In 977 Boleslav again attacked Bavaria, but on this occasion was barred from annexing any lands by Otto II. Boleslav's reign is mostly notable for the foundation of the bishopric of Prague (973) which was placed within the jurisdiction of the Archbishop of Mainz. In 982 Vojtech (later known as St Adalbert of Prague) was appointed to this position until in 994 he abandoned his primacy to lead a mission to the Prussians. Adalbert was martyred in Prussia on April 23 997 near Gdansk and his remains became the holiest relics of Poland. War between Poland and Bohemia was continual in this period and by 990 Boleslav had occupied Silesia. Boleslav on September 28 995 stormed Libice in southern Bohemia and massacred the Slavnikovci. This clan had been the main rival of Premyslid power in Bohemia. Boleslav's brutal triumph esured the unity of Bohemia proper under a single ruler.