Challenge of the Three is the last book in the 3 Girls, 3 Pantheons series.
Reya Milan- Greek daughter of Hyperion
Amalia Ingard- Norse daughter of Thor
Emma Halman- Roman daughter of Gaia
Adro- African god of Destruction, evil side of Adroa
The three girls have to batlle Adro, the African god of destruction, so he doesn't destroy the world. If they fail, the world will be destroyed and mankind as we know it will vanish. The reason that African demigods cannot battle Adro is that he cannot be killed by any African demigod. (that includes Egyptian, as Egypt is in Africa) Each pantheon of gods chose a demigod that once saved the camp. Reya saved the camp by returnin a ring to Zues, as he would otherwise be very mad and put his wrath on Camp Half-Blood. Emma saved Camp Jupiter by killing Gaia and defeating the giants and Earthborn. (A/N- I have not finished the other story yet. She has yet to save her camp.)