Percy Jackson and Heros of Olympus - Seven Becomes Eight

Percy and Annabeth have just returned from Tartarus, the place where monsters go after a hero defeats them. But, the Great Prophecy can't be put on hold. Rachel the Oracle visits Percy in a dream and recites the prophecy:
Seven half-bloods shall answer the call
To storm or fire, the world must fall
An oath to keep with a final breath
And foes bear arms to the Doors of Death

Pausing for a fraction of a second, Rachel finishes the last few lines of the prophecy that for so long thought completed:
Seven will become eight
And change the world's fate
If they succeed the earth will plead
And these heroes will have completed a great deed
But the eighth can be only a half-blood of the Big Three
Or else their quest will not succeed

This can only mean one thing: Something devastating is about to happen, and its going to either happen now or in another lifetime. And with Chloe's rotten luck, of course she, believing she was a mortal for the fifteen years of her life, gets dragged into it.