Casimir II the Just
In 1186 Leszek, Duke of Masovia died without issue. In his will, he left all the Masovian-Kuyavian principality to his youngest uncle Casimir II the Just. However, Mieszko III could mastered Kuyavia; soon after, he passed his new acquisition to Bolesław (some historians believed that Bolesław only obtain Kuyavia in 1194 after the death of Casimir II the Just).