Niels Olof wrote:As for the spell book, it does raise questions of its own, such as a) why would you develop such a spell, and once its potential for devastation was realized, b) why would you still keep it around? Then again, the Equestrians have a dubious security record at best when it comes to powerful magical artifacts (
Magic Duel, Elements of Harmony, ...).
I can't say for certain who made the spell or the book, but it certainly doesn't seem to match the style of the princesses, so I highly doubt they made it. I'd imagine that they likely took it to stop the
previous owner. That doesn't really answer the first question, I'll admit, but I doubt we'll ever learn, and in the end it doesn't really matter.
As for why it's kept around - Celestia and Twilight both copied a bit of Sombra's magic during TCE, so it might be that the princesses would rather hang onto such artifacts and study them rather than outright destroy them. Considering that eating the book didn't stop Rarity, it might also have been that destroying the book wouldn't really help.
Don't really know where I'm going with these answers, just kinda throwing those ideas out there.
About the security, though - the book was kept on a dangerous staircase, behind a locked gate, behind a hidden door activated by pulling a certain book
in the middle of a library, in a castle
where the princesses lived, in the middle of the Everfree Forest. It's not exactly a place that any random pony is likely to come across; in the past, that would've been an extremely secure spot, and in the present, most ponies don't even know the book exists. Very few ponies would be looking for it in the first place, let alone know how to find it.