I see a lot of people saying the twist is that the heroine, Elara, was secretly the lost princess all along. That's the surface-level shock, sure, but the real gut-punch is how she finds out. It's not some grand recognition ceremony. She's hiding in the enemy lord's library, reading his private journals about the royal family he helped slaughter, and she sees her own childhood nickname—'Little Thorn'—scribed next to a description of the infant princess's birthmark.
She realizes the man she's been plotting to kill, the one she blames for her entire miserable life, is the very person who smuggled her out of the castle and hid her in plain sight. The 'veil' wasn't just her disguise; it was the narrative of vengeance he constructed to keep her alive and driven, making her strong enough to eventually reclaim the throne, even if it meant she'd hate him forever. The twist isn't her identity, it's his motivation. Totally reframes every cold interaction they had.