Old Prussians
In 996, Duke Bolesław I of Poland sent the longtime Bishop of Prague, Adalbert, to Christianize the Old Prussians. He was martyred by the Prussians for his efforts in 997. Bolesław I, who had bought Adalbert's body from the Old Prussians for its weight in gold, had Adalbert laid to rest in the cathedral at Gniezno, which eventually became the ecclesiastical center of Poland. Otto III and Bolesław I worked together to canonize Adalbert, making him the first Slavic bishop to become a saint. In December 999, Otto III left Italy to make a pilgrimage from Rome to Gniezno in Poland to pray at the grave of Adalbert.