The Flames of Hope (Wings of Fire, Book 15)

"Luna has always wanted to change the world--to fix it, to free it--even if she's never actually known how. Now that all of dragon- and humankind are in mortal danger, Luna is flying back home to Pantala with a team of dragons on a rescue mission, determined to be brave and useful. But saving a continent isn't as easy as a prophecy makes it sound, and "facing a great evil" definitely requires more than the fiery silk that Luna is uniquely able to create. As she fights her way to the abyss that hides the dark roots poisoning Pantala, Luna must uncover a long-buried secret and unite her friends, her enemies, and her own powers. If she doesn't, she won't get to change the world. She'll have to say goodbye to it--forever."--