Casimir III of Płock
Casimir III of Płock (pl: Kazimierz III płocki; 10 June 1448/8 June 1449 - 9 June 1480), was a Polish prince member of the House of Piast in the Masovian branch. He was a Duke of Warsaw, Nur, Łomża, Liw, Ciechanów, Wyszogród and Zakroczym during 1454-1471 jointly with his brothers (under regency until 1462), Duke of Płock, Płońsk, Zawkrze and Wizna during 1462-1471, and after the division of the paternal domains in 1471, sole ruler over Płock, Wizna, Wyszogród (until 1474), Płońsk and Zawkrze until his abdication in 1480, and Bishop of Płock during 1471-1480 (as secular administrator until 1475).
He was the fifth son of Bolesław IV of Warsaw and Barbara Olelkovna of Slutsk-Kapy, a Lithuanian princess, (granddaughter of Vladimir Olgerdovich). The premature death from three of his older brothers during 1452-1454 left him as the second surviving son of his family.
Life
After the death of his father on 10 September 1454, Casimir III and his siblings where place under the guardianship of their mother Barbara and Paweł Giżycki, Bishop of Płock. The regency ended in 1462, when their older brother Konrad III attained his majority and became in the legal guardian of his younger brothers.
Thanks to efforts of Princess Catherine of Płock, in 1462, Casimir III and his brothers where able to obtain the districts of Płock, Płońsk and Zawkrze, although they lose Belz, Rawa Mazowiecka and Gostynin, who where formally incorporated to the Kingdom of Poland.
Casimir III and his brothers formally attained their majority on 3 April 1471. In the division of the paternal domains, he received Płock, Płońsk, Wizna, Wyszogród and Zawkrze. Despite he obtain his own domains, Casimir III was prepared to pursue a Church career; for this purpose, even in his early boyhood, he managed to occupy the position of Dean and Canon of Płock Cathedral.
After the death of Ścibor z Gościeńczyc, Bishop of Płock, on 4 May 1471, Casimir III announced his candidacy for the post. Unfortunately, he was opposed by King Casimir IV, who chose his Vice-Chancellor of the Crown and Royal Secretary Andrzej Oporowski as successor, while the Chapter supported their own candidate, Ścibor Bielski. However, when the Chapter realized that they can't imposed their candidate, decided to support Casimir III. The case was finally settled in Rome, where, despite the vigorous intrigues of the Polish King, the Pope finally decided for Casimir III, thanks in part for the major support who received from Frederick III, Holy Roman Emperor (who through his mother Cymburgis was related with the Masovian Piasts). The election of Casimir III took place despite he didn't had the legal age for a canonical post - 30 years. For this, he ruled the Bishopric only as a secular administrator until 10 September 1475, when, after the grant of a Papal dispensation for his age, Casimir III was finally consecrated Bishop of Płock at Pułtusk Church.
After his consecration, Casimir III resignated his paternal domains to his brothers: Konrad III obtain Wyszogród and Janusz II the rest of his lands.
Casimir III wasn't able to be a prominent Bishop. Winning the episcopal office cost him large sums of money, which is now trying to recover from the Church Treasury, causing the understandable opposition of the Chapter. The case went up to the Pope, who helped the diocese taking his finances under his direct control. Soon after, Casimir III died of the plague on 9 June 1480 at Pułtusk, where he was buried.