Age of the amulet

"Jim travels back in time and encounters Gogun the Guardian, hailed as the bravest Trollhunter who ever lived. However, Jim is shocked to discover that Gogun is inept, unskilled, and terrified of everything and everyone. Jim knew when he was chosen as Trollhunter he would have to save mankind in the present, but he never realized he was responsible for the past as well. The fate of the world is a lot of responsibility for a fifteen-year-old--but luckily Jim is up to the task; as long as he's home in time for dinner"--Provided by publisher.