User blog comment:SayuriDarling/The Blood of Olympus Official Discussion/@comment-25540899-20141015084002/@comment-24055022-20141015141436

I'm not sure what the sacrifice is either, but Annabeth causing Percy the most problems as in the Mark of Athena because she went on that solo quest, but Percy didn't want her to go, and he wanted to stay with her, but he couldn't. So they kind of made it hard for each other.