Dragon's Gift: The Valkyrie, book 4: Attack by MagicLinsey Hall2018

Welcome to the Magical Death Match. For five long years, I’ve wanted just one thing—to find my sister Rowan. Her disappearance tore a hole in my life. Now I’ve found her—but she’s a captive of the same Rebel Gods who hunt me. They’ve enchanted her to follow their every whim, which will make rescuing her nearly impossible. But I’ll do anything it takes, including infiltrating an ancient Celtic war camp. I’ll even join the death fights in their arena. To succeed, I'll need to learn to use my new magic. Thankfully, I have the sexy Celtic war god Cade on my side, along with my friends at the Undercover Protectorate. Thank fates for that, because I'm going to need all the help I can get if I want to survive.