thewizardguy said
The Book of Peace seems to only be her 'weakness' because she wills it. She isn't particularly impeded when it's opened, and only uses it as some kind of mcguffin. Something to give the mortals a chance, and for her to take away. I believe it's quite likely that the mark on her chest might be another one of those self-imposed limitations. Also, she's a Goddess. Literally. She might not be omnipotent, but she isn't going to have one definite weakness per se.
The Book is a tricky topic, while she does will it to be her weakness it also gives Syracuse and the other cities a sort of magical energy. It's difficult to explain but after the Book is shut the city begins to crumble, which is exactly what Eris wanted. I've believe that the Book could like a good luck charm, it's enchantments kept peace in whatever city housed it, I also thought that it might be more like a security blanket. The 'cross my heart' mark while it might be a self-imposed limit it also would make sense that if you look at it from the eyes of a child. In kid's movies the villain usually has a crucial weakness, as a kid it made sense to me that the promise of a deity is absolute. If you want to look at it from a mythological point, Eris could have sworn on the River Styx. Meaning she had to keep those particular promises, however explaining that to children is a lot harder than using a familiar mark of a promise and saying she has to do it. We must also take into account that while all of what you said could be true neither of us has much proof either way. All we have are the events of the movie, it's hard to say whether Eris set up the whole thing for fun because we just don't know. It adds layers of complexity that the common 7-11 year old can't fully understand.
I can't help it either, sorry.