Spoiler: Chapter 7 Page 6
Lots to parse on this page.

My reading of it is that Ed's parents sealed themselves away somewhere to prevent some kind of powerful spirit from escaping? And that they might've had some hand in causing it to become so powerful in the first place?

Speculation: I'm willing to wager that this grudge spirit the Burgers trapped is either the shadow creature that later goes on to possess Spender, or the "strongest spirit" that Nin claims he defeated. Those might actually be one and the same, now that I think about it.

Boss Leader and Ed's mom certainly seem like they are/were more than just work colleagues. And I appreciate that BL continues to make cracks in the fourth wall, even at the expense of the subtitles.