Been thinking lately that maybe the prophecy is the fall of the Kingdom and Diana is actually its savior, but the prophetess is reading it wrongly. And that's why we get to see the part of the Prophecy, most of the King's attendants actually take her side.
The prophecy is that the one who ruins the kingdom comes from the forest.
We don't know who is Diana's father, but The Sea tells Julian that Diana's special because "she carries another one within her".
Not only that but The Sea also calls Diana her daughter.
So maybe Diana's father was someone blessed by The Forest, that would make Diana a carrier of two blessings.
Anyway, the King is probably going to kill Julian and Diana will bring the kingdom down, not for revenge but for the good of the people or something like that.