About that time the "executionist movement" (Polish: "egzekucja praw"--"execution of the laws") began to take form. Its members would seek to curb the power of the magnates at the Sejm and to strengthen the power of king and country. In 1562 at the Sejm in Piotrków they would force the magnates to return many leased crown lands to the king, and the king to create a standing army (wojsko kwarciane). One of the most famous members of this movement was Jan Zamoyski. After his death in 1605, the movement lost its political force.
what did the Sejm in Piotrków forced the magnates to do?
return many leased crown lands to the king,