United as One by Pittacus Lore (2016) HarperCollins

The seventh and final book in the original Lorien Series (There are additional books since, but they seem to be discarded chapters from different points of view).  In this episode, number eight is a prisoner, but number four, John, gets him to participate in a revolt against the Mogodorians, who think they’ve one.  John discovers he can learn any legacy and goes with the others to the mountain headquarters in West Virginia.  Ra has become something more fluid than ever before and his protege, also thinks she is unbeatable.  Both are wrong as Adam tumbles with her into an abyss, taking her from the formula and number four discovers Ra’s weakness is to be healed.