User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-68.185.66.252-20130609025641/@comment-3146930-20130609030719

As far as who is staying behind, I think it is Frank. The end of The Son of Neptune has Frank talking about how Mars left him a book called The Art of War, which talks about the price of war and sacrifice. Mars also told him that Gaea can't really control him the way she can with the others and in a way she fears him. Obviously because of Percy's flaw, he would want to stay in Tartarus if it meant saving his friends. Frank on the other hand may prevent him from doing so and do it himself. Maybe he will knock Percy out or something, lol. It just seems like Frank's theme is sacrifice.