A War With Fiends - The Essence Chronicles, #3

Essence is on the brink of war. Essie must mitigate all conflicts before the fiendish tyrants in the Land Above notice the chaos afflicting the realm. After barely escaping a sudden attack, Essie doubts she'll have the strength. And with so many humans mistaking her as the enemy, convincing them to heed her warnings may be an impossible feat. Essie's allies explore the past hoping to find a solution for the present.
Instead, they uncover secrets that will alter the future of the ultimate crusade against the Land Above. But if Essie and her friends can't stop the fighting in Essence, everything and everyone in the realm will be extinguished from existence, ending the war with fiends before it can even begin.