The Mythical Hero's Otherworld Chronicles: Volume 7 by Tatematsuri

The Black Prince has fallen, cut down in battle against Six Kingdoms. As news of the tragedy sweeps the empire, Liz and her allies resolve to set aside their grief and finish what Hiro started. Yet the new heir apparent faces a treacherous path to the throne. In the palace, forces conspire to undermine her rise, some driven by mundane self-interest, others by more sinister motivations. On the field, she must contend with Luka du Vulpes, grievously wounded by her clash with Hiro but all the more dangerous for her scars. And just when the stage seems set for a decisive battle, Queen Claudia of Lebering steps into the fray, accompanied by a mysterious new confidant: a masked man with black hair who identifies himself only as Surtr...