Man, I just finished 'Veil of Fate' last night and I'm still reeling. So the big twist is that the protagonist, Kael, isn't actually the chosen one prophesied to save the kingdom—he's the unwitting cause of the very doom he's trying to prevent. His supposed heroic destiny was fabricated by the royal astrologer to manipulate him into gathering the three magical artifacts, which, when brought together, don't seal the ancient evil but instead awaken it from its slumber.
What makes it so gut-wrenching isn't just the betrayal, but that Kael's own innate magic, which he thought was a blessing, is the leftover life-force of the imprisoned god, slowly leaking out. Every spell he's ever cast has been weakening the seal. The final act where he has to choose between sacrificing himself to re-bind the evil or letting the world burn to preserve his own stolen existence… yeah, that kept me up. The book really makes you feel every ounce of his shattered identity.