The Dark (Guardians of Time)

Isabel and Ethan are joined by Isabel's brother Matt, who is still getting accustomed to life in the Guard. This time, they must find Arkarian, their ageless mentor, who has been kidnapped and sent to the underworld. Although Isabel and Ethan have been forbidden to rescue him, an ever-increasing connection to Arkarian drives Isabel to convince Ethan and Matt that they must risk everything to find him despite the potential consequences.