Sapphire said
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.
Good points, all ^^ However, I like to theorize on the blank spots in movies, esspecially those I like.
Personally, the biggest mystery to me is what exactly the Book of Peace is. Who made this thing? Where did it come from? How does it work? It's obviously quite powerful, as demonstrated by the big shining lights, but who has that kind of power, and doesn't like Eris? Personally, it seems to me that it's most likely that Eris is, in fact, the creator of the Book of Peace. She's the only entity of enough magical power we see that could create such a powerful relic, and it fits in with the idea that she's just playing games. She isn't seriously trying to spread chaos, she's just faffing about in the human world.
I understand that she used the familiar mark to identify with younger children, but the idea remains the same. This plays out a lot like an old mythological tale, in which the Gods are bound by their promises. However, Eris appears to be nearly omnipotent, capable of doing anything she wishes. I think that the self-imposed limit is the most likely option here, based on what we see of her power and personality.
