Daten no Inugami: Slash Døg - High School DxD Universe

Set in the same universe as High School DxD, but years before the events of that series, the story centers on Tobio Ikuse, a high school student still reeling from an incident that caused the disappearance of his childhood friends. One day, he is attacked by beings known as "Utsusemi," but is rescued by Natsume, one of his missing friends. Together with Natsume and another girl named Vali, he races to find out the cause of his friends' disappearance.

(Source: ANN)