User blog comment:BlueLantern1995/The Prophecies: What We Know and What is Logical/@comment-5562903-20121112191007/@comment-74.134.155.110-20121113045110

This is a very real possiblility. Then again, the oath can be made to anyone from anyone. Percy promised he would fight for Camp Jupiter at the end of Son of Neptune and we're coming up to a war between the two camps. I believe it was mentioned in MoA that Aphrodite and Reyna talked about something (or else it was hinted), and for all we know there may have been a promise involved.

It may not even be in the past books. It may be in a future book. We might not even see this part of the prophecy fulfilled in House of Hades - this is still planned to be a five-book series, as far as I'm aware.