Man, where do you even start with that one? Book five is just twist after twist, honestly it almost felt like too much at points. The biggest one is obviously the royal bloodline reveal – it turns out the entire premise about the usurper king being the hero's father was a massive lie concocted by the seers to prevent a prophecy they misread. Instead, the true heir is actually the side character who's been with the party since book one, the one everyone thought was just comic relief. But for me, the real gut-punch was learning that the 'guide' figure, who'd been giving them visions throughout, was actually a manifestation of the main villain's fractured consciousness trying to steer them toward self-destruction.
That part genuinely re-contextualized the entire series. It makes you want to go back and re-read every interaction with that character, looking for the subtle manipulations. I've seen some people argue it was a cheap trick, that it breaks the internal logic, but I think the breadcrumbs were there if you paid attention to the wording of the prophecies. It's less about a hidden identity and more about the nature of free will versus predestination, which the series has always played with. The fact that the twist changes your understanding of the past books instead of just the current plot is what makes it work for me.
Of course, there's also the smaller, more personal twist that the protagonist's love interest wasn't actually captured at the end of book four – they willingly defected, having been a double agent the whole time. The letter revealing that was brutal to read.