The Unwanteds Quest: Dragon Captives by Lisa McMann

My name is Thisbe, and my twin sister and I are the most powerful mages in Artime. However, Alex doesn't trust Fifer and I enough to teach us how to properly cast spells. I understand he's upset because we're a little reckless at times, and people are afraid I might lose my temper and kill them. We need to convince Alex that we can do something good with our powers, but our plan will be risky. The dragons need help, so we'll sneak off and save them ourselves. However, we now realize we may be in over our heads. No one in Artime knows where we've gone, and the people of Grimere want to capture us as thieves. We don't know who we can trust, since we've never left Artime. We must reach the castle at the top of the mountain and hope it doesn't put us in more danger. I'm kind of sure we can succeed, but there's also a good chance things will go terribly wrong. As we near the castle, I'm terrified to discover my worst fears are coming true.

This book is the first in a spin-off series, and it might help to read The Unwanteds first. It's not 100% necessary, since this book reviews most of the major events from ten years before. Two-year-old Thisbe and Fifer ended the war in that series by unintentionally displaying their immense powers, and bad things still tend to happen when they get angry. This explains why they still frighten everyone else. They have huge internal conflicts going on in their minds, as they want to do good but don't have the experience to foresee the consequences of their actions. So how can they prove themselves if they're never given the chance or training? Simber was a major character in the original series as a protector of Artime and Alex's confidant, and he continues that role in this book. The giant, flying beast made of stone is key in saving the twins from their imprisonment. Other familiar characters are included to help readers who are adjusting to this spin-off series.