Quintessence (Elementals' Challenge, #5)

One final Challenge looms. Two hearts carry the seed of victory...if they survive.
Tremors and distant screams rattle Eliane awake in the dead of night, but they aren't coming from the ground. They're rumbling through her mind, triggering the supernatural connection between her and her best friend, Joao. A connection they've shared since they journeyed to Argentina as children—and came away with something that makes them more than human.
Joao knows something disturbing has awakened his and Eliane's naguals—mysterious symbiotes they each carry inside them. One of the four Elementals has suffered defeat in their initial Challenge against their Demonos counterpart. When an Elemental is defeated, all four must fight in final Challenge. A battle in which he and Eliane have some part to play.
Joao's first instinct, to protect Eliane at all costs, wars with their mutual promise to resist the attraction between them, at least while the symbiotes' power is untapped. But as attacks escalate, they begin to understand this Challenge isn't about who will have the power of life or death over mankind. Defeat will spell total destruction of all humanity, and time is running out on Joao and Eliane's one chance at forever.