Daten no Inugami: Slash Døg - High School DxD Universe  

堕天の狗神 -SLASHDØG- ハイスクールD×D Universe

In a world parallel to High School DxD, before its events took place, Tobio Ikuse is a high school student grappling with an incident that led to the vanishing of his childhood companions. One day, he encounters "Utsusemi," who attack him. However, Natsume, one of his missing friends, rescues him. Alongside Vali, they embark on a quest to uncover the reason behind their friends' disappearance.