Daten no Inugami: Slash Døg - High School DxD Universe  

The Fallen Dog God, 堕天の狗神 −SLASHDØG− ハイスクールD×D Universe

Four days after missing his school trip on a luxury liner with his classmates, Tobio Ikuse, a second-year high school student, was devastated when he heard about the disappearance of his teachers and classmates from his school. Despite slowly recovering from the sudden loss of his friends, Tobio was shocked when he accidentally encountered one of them again while chasing after his missing friend Sasaki. However, his joy quickly turned into horror when he saw Sasaki alongside a Lizard Monster that attacked him together with the monster. Cornered by the monster, Tobio was rescued by another survivor of the missing luxury liner, Natsume Minagawa, who handed him an egg before parting ways. That night, Tobio was under attack again by two people with monsters. Amidst all the chaos, a Puppy emerged from his shadow to rescue him. After fending off the two monsters, Natsume approached him once again before deciding to take him to their hideout.

Daten no Inugami: Slash Dog is a reboot of the original Slash/Dog novel, and takes four years before the events in High School DxD.