The great 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.

Unraveling the prophecies:

1) The first line obviously states that seven half-bloods would save Olympus, and in this case, from Gaea, Mother earth. The seven half-bloods would be: Percy Jackson, Jason Grace, Leo Valdez, Piper McLean, Hazel Levesque, Frank Zhang and Annabeth Chase.

2) Storm would lead us to believe they mean Jason Grace (As Jason can control the winds) or Percy Jackson (Percy can control water, rain, that includes a storm), Fire= Leo Valdez. (Leo can summon fire), Piper thinks that it would either be Jason Grace or Leo Valdez who stops Gaea and saves Olympus, while the others would suffer.

3) This sentence is still quite unclear, as the time is unknown. Piper would suggest that it was going to be Leo Valdez who makes an oath as he dies.

4) Gaea opens the Doors of Death so that the evil monsters would be able to rise and won't go back to Tartarus so easily. And since Gaea is in control of the Doors of Death, she is able to allow the evil to stay at her side while demigods won't stay.