I read a manga a few years back but stopped part way through and now I can't remember it's name or author to go back to it. It had a very dark tone but I can't remember the central plot.

There was a side character that practiced taxidermy and had secretly done so on humans. There might've also been a setting that was a house made of many rooms/boxes that one person was not allowed to leave and was involved in human trafficking. Another of the side plots might've had to do with a mermaid show where the stage makeup was poisonous so many of the stars were young because the older women's looks faded quickly due to regular use of the makeup.

One of the characters was an orphan and they went to the shanty town where he was raised. A part of the plot had to do with obtaining people with special blood/ fighting powers I think. It had a semi historical Japanese aesthetic/setting. That's all I can recall of it. I could be getting some of the details wrong but any help in finding it again would be appreciated.

The mermaid, child trafficking and blood power seem to remind me of Adekan but I can't say for sure since its been years since I last read it,

I think that's it! Thanks ?