User blog comment:Godisme/Sheathebreaker/@comment-5044191-20110629184208/@comment-1351982-20110629213226

Yea, I was thinking the same exact thing when I saw the mansion. Someone mentioned this on one of the forums (forget which one), and they suggested that the Fullbringer arc is ending soon, and the reason we have not seen the mystery Shinigami yet, is because it will lead to a new arc. I think this actually makes a lot of sense, because Kubo has been clear in many interviews that he will explain Isshin's past. The unknown Shinigami might possibly be the lead-in (Plus Kubo said this arc would only last about 8-10 months).