The joke here, I think, is that Master-kun is every gacha protagonist.

Or something.

Mostly going off the comment on the fanbox version of the post:

已知嫌犯已掌握靈子轉移術式，為保護你各位御主的人身財產安全，入夜後請盡量不要避免獨自外出。以下為目擊情報。