Welp, guessing this is where leslie's "black saint" powers are gonna come into play to stop the box's effect(since the queen dowanger burned the records, we don't know the method they used to not kill the arabella if there was one).
@Cleesus
She is from a foreign nation that the empire conquered, it's not really hard to figure out.
And the arabella is going to die, that's basically what she was saying, if leslie dies it will save the empire but the dutchess would lose her daughter, if eli get's it she dies but the queen will get the box and thus a powerful weapon (guessing the arabella dies to surpress the curse for a time).
@Indomiegoreng
As i said to Cleesus she is from a nation the empire conquered(or annexed, can't remember the exact scenario but i know she was part of a arranged marriage to prevent full annexation of the country or something), as such it could be simple revenge, or it could be she wants her son on the throne to turn the empire into a puppet of her home country.