User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-6951062-20130321230637/@comment-4649443-20130322014928

But Percy has to make an IMPOSSIBLE sacrifice, and there are only three people(and two monsters) I think are  impossible for him to sacrifice:

People:

Annabeth

Tyson

Mom

Monsters:

Mrs. O' Leary

Bessie

And since only Annabeth is with him it might be her.