Speaking of the books building up to something, we have the stake and other details which are important, but I also remember The Fates talking to Anastasia at the end of book 3 where they mention how she's something they've never seen before and how her threads are different or unique, I can't remember exactly what. I could be wrong but between Wilfred's vision of the stake, what the sirens said about fate not being good or bad, Wilfred's parents making a comment about Anastasia, and some other things, I feel like we might get to see clues about what's coming based on Wilfred's visions and that Anastasia is going to play a big role in whatever is coming (and not just because she's the main character lol). Maybe something like she as the only human has a power outside fate that none of the rest do or can do something the others can't because they are bound by it.