OK, y'all. I'm trying to find a book I read many moons ago. I feel like it was by Diana Wynne Jones, but it's not in her bibliography. Massive spoilers incoming, obviously, but I can't remember what the spoilers are for.
The book starts on an island nation in the south of the world, with a rigid code of conduct which one of the main characters is being disciplined for breaking. The main characters leave on a quest to the oppressive and powerful kingdom in the north, and its revealed that one of the other main characters is the crown prince of the evil kingdom in the north, and can use their magic. If I recall correctly, his use of that magic makes dark veins stand out under his skin, and he has to fight against it controlling him. There's some kind of time limit, I think if he uses the magic too much, it'll take him over and he'll become the new ruler.
To gain some advantage over the evil kingdom, they visit an abandoned city, break into some kind of temple, and have an encounter with some kind of deity, which might then take over one of the characters?
Later in the story they make it to the evil palace, and there's a plotline about multiple children of the evil king trying to kill this guy, so they can inherit the throne. I think the evil palace is embedded in a mountain somehow.
Anyone who can set me on the right track, it'd be much appreciated!
Google Gemini sez:
The book you're describing is most likely "The Shadows Between Us" by Tricia Levenseller.
Here's why it matches your description:
Island Nation in the South: The story begins on the island of Iolara, a tropical island with a strict code of conduct called the "Way." Discipline and Quest to the North: The protagonist, Alessandra, is being punished for practicing magic, which is forbidden. She joins a group on a quest to the tyrannical Empire of Aethel in the north. Crown Prince with Magic and Internal Struggle: One of the main characters, Kallias, is revealed to be the crown prince of Aethel. He possesses forbidden magic that manifests as dark veins and threatens to consume him if he uses it too much. Abandoned City, Temple, and Possession: The group ventures into the ruined city of Aethel's former capital and break into a temple. There they encounter the Shadow Queen, an ancient deity who temporarily possesses one of the characters. Evil Palace and Power Struggle: The climax takes place in the Obsidian Palace, carved into a mountain, where Kallias faces betrayal and assassination attempts from his siblings vying for the throne. Although some minor details might not perfectly align with your exact memory, the core plot points you mentioned strongly suggest "The Shadows Between Us" is the book you're looking for.
Unfortunately, that's definitely not it. I've never read anything by Tricia Levenseller. None of the other names align either. :( Thanks for the input though!
After reading a couple of chapters online, I think I was lied to yet again by AI. Sorry for the wild goose chase.