What is the main conflict in The Power of Six by Pittacus Lore?

The Power of Six by Pittacus Lore is the second volume in a series that began with the book "I Am Number Four." It is a saga that pits good against evil on an interplanetary scale. Nine children and their guardians (Cepans) have been sent to Earth from the planet Lorien which is under siege. The Mogadorians have used up their own planetary resources and are now expanding outward, seeking greener pastures to sustain them. Lorien, the closest inhabitable planet to them is their first target. In an effort to save their race, nine children entrusted with the gifts of the elders are sent away in the hopes that when their Legacies develop, they will be strong enough to return to Lorien and reclaim their birthright. A tenth child, just a baby at the time, is sent to Earth on a second flight with a ship loaded with Chimaera.