User blog comment:B214/General Discussion/@comment-30924344-20171227185455

In the Afterword of V23, Ishibumi said "The story of the 100 ghosts and Millarca will be published in the future". So in this volume, it seems info was revealed involving an incident between Millarca and a hundred ghosts.

Got the info from Baidu: https://tieba.baidu.com/p/5146672382?pid=116558512724&cid=&red_tag=2379616579#116558512724