With Otto III's approval, Stephen was crowned as the first Christian king of Hungary on Christmas Day, 1000. Congress of Gniezno 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. Otto III's pilgrimage allowed the Emperor to extend the influence of Christianity in Eastern Europe and to strengthen relations with Poland and Hungary by naming them federati ("allies"). On the pilgrimage to Gniezno, the Emperor was received by Bolesław I at the Polish border on the Bobr River near Małomice. Between 7 and 15 March 1000, Otto III invested Bolesław I with the titles frater et cooperator Imperii ("Brother and Partner of the Empire") and populi Romani amicus et socius ("Friend and ally of Rome"). Otto III gave Bolesław a replica of his Holy Lance (part of the Imperial Regalia) and Bolesław presented the Emperor with a relic, an arm of Saint Adalbert in exchange. On the same foreign visit, Otto III raised Gniezno to the rank of an archbishopric and installed Radzim Gaudenty, a brother of Saint Adalbert, as its first archbishop. Otto III also established three new subordinate dioceses under the Archbishop of Gniezno: the Bishopric of Kraków (assigned to Bishop Poppo), the Bishopric of Wrocław (assigned to Bishop Jan), and the Bishopric of Kołobrzeg in Pomerania (assigned to Bishop Reinbern). Bolesław I subsequently accompanied Otto III on his way back to Germany. Both proceeded to the grave of Charlemagne at Aachen Cathedral, where . Both arranged the betrothal of Bolesław's son Mieszko II Lambert with the Emperor's niece Richeza of Lotharingia. Final years Return to Rome The Emperor spent the remainder of 1000 in Italy without any notable activities. In 1001, the people of the Italian city of Tibur revolted against Imperial authority. Otto III besieged the city and put down the revolt with ease, sparing its inhabitants.