Mouryou no Hako 7
Well, that was one of the most esoteric info-dumps ever. All I got out of that mouryou are a crazy folklore creature with all kinds of conflicting definitions.
Is the box/four walls thing why the number 4 is considered unlucky in Japan?
4 (shi) is like death (shinu). I think that's all there is to it, though the box/grave similarity doesn't hurt.
I rewatched that episode again closer to the end of the series, and while my analysis rests on some later stuff too, I think there are a few key things to take away from Kyougokudou's longer discussion.
- Mouryou are natural spirits, but nobody really knows exactly what they are.
- Mouryou are closely related to the dead, and are known to take away dead bodies.
- Mouryou thrive on the thresholds between the known and the unknown, between the world of the living and the world of the dead.
It'd be one thing if they were easy to understand, but they defy comprehension to some degree, and that's part of what's dangerous about them. To know what they do, we need to find the portals they inhabit, and see what happens to those surrounded by them. If this priest has somehow grasped the true nature of the mouryou, then how do they drive people to unhappiness?