A papal legate and diplomat, active in the Christian conquests of Prussia and Livonia.
William was probably born around 1180 in Piedmont. He served as papal vice-chancellor (1219/1220-1222), bishop of Modena (1222-1233/1234), and cardinal-bishop of Sabina (1244-1251).
William first acted as papal legate for Livonia in 1225-1226, where he reached a temporary solution to the territorial disputes between Danish and German crusaders. After serving as a diplomat in Pomerania and Silesia (1229-1230), he returned to Livonia as legate in 1234-1235 and reorganized the structure of the Livonian bishoprics. He also mediated a settlement between Conrad, duke of Mazovia, and the Teutonic Order over the tenure of the lands formerly held by the Knights of Dobrin. In 1236-1238 he arranged the restitution of North Estonia to the king of Denmark, and subsequently (1239-1242) he was in Prussia where he regulated relations between Bishop Christian and the Teutonic Order and established the Prussian diocesan organization that was recognized by the pope in 1243.
In 1244 William was again named as legate for Prussia but actually stayed in Lyons. In 1246 he was appointed as legate for Norway and Sweden; he crowned Haakon IV Haakons- son as king of Norway in 1247, and on his way back from Sweden to Lyons he took part in the coronation of William of Holland as king of Germany. William of Modena played an important role in the politics of the crusade regions of Livonia and Prussia, where he was able to successfully resolve complex political conflicts. He was often appointed legate at the request of local powers and could usually rely on them in his activities; his regulations often favored the Teutonic Order. He also assisted the expansion of the Dominican Order in the Baltic region.