To commemorate the 10th anniversary of the content of "The Idolmaster Million Live!" a common costume implemented in the game "Theater Days" was introduced. The costume name "Re-Prologue-X" is read as "Re-Prologue-Cross".
On January 14, 2023, it was first unveiled when the 10th anniversary visual was released at the live event "THE IDOLM@STER MILLION LIVE! 9thLIVE ChoruSp@rkle!!" DAY1.
