User blog comment:Jopjopjop/Volume 20 Spoilers/@comment-24421137-20150719030548/@comment-4320546-20150719031838

The King's Monument. According to Rias, High Class Devils must register at the Monument in order for them to officially be recognized as a King. It was because of that monument that no one realized the existence of the King Piece and it was used as a psuedo-King piece itself.