The Fighters Book 3: Son of Thunder (Forgotten Realms)

It was the best thing that ever happened to him. It was his god’s blessing. It was hell. Vell was content to be a mere warrior in the Thunderbeast tribe, staying behind on the hunt to guard the camp. But then something alien awakened deep within him, the spirit of a behemoth that he could not control. With it came attacks from the sky, visitors from far lands, and a mysterious command from their ancestral totem: Find the living. And this time, no one was going to let him just stay behind. Reptilian creatures known as Thunderbeasts, kept secret by an ancient magic, dwell in the High Forest. But when the Zhentarim finds out about the powerful magic, the Thunderbeasts' days of seclusion are numbered