I Got a Cheat Skill in Another World and Became Unrivaled in the Real World, Too, Vol. 6 (light novel‪)‬

Yuuya has obtained the dark power of the Demons, despite being the disciple of one of the deities who protects the world. To learn to control it, he begins a harsh training regimen. On Earth, Ousei Academy goes on summer break, and that means Yuuya and his friends make plans to have the time of their life at the beach. Of course, that means he’ll be surrounded by beautiful girls in swimsuits—turning up the heat on the beach! Meanwhile in the other world, a battle between Yuuya and the peerless sword princess, the Deity of Swords, is being arranged. But their contest is interrupted when the Apostle of Atrocity suddenly attacks, planning to destroy humanity. Now, the fate of the world is in Yuuya’s hands!