User blog comment:SayuriDarling/The Mark of Athena Official Discussion/@comment-111.193.219.138-20121008151352

Some crazy theories:

Leo said that someone would need to be trapped on the Tartarus side of the doors of death in order for it to close, which might be foreshadowing what could happen, and Mars also told Frank that Percy would have to make a great sacrifice in the end. What if the sacrifice that Percy would have to make is leaving Annabeth on the Tartarus side? Obviously Percy wouldn't do it, he promised Annabeth he would never leave her again. He would trap himself in Tatarus with her, hence an oath to keep with a final breath. Or maybe the oath is Nico's promise to Percy to lead the remaining five heroes to Epirus, and Nico gets killed on the way.