The Party of Heroes
Issei and the others continue their training, even while they fully enjoy themselves in Kyoto. Issei's group meets the mysterious nine-tailed fox once again and, this time, she begs them to help the youkai save her mother from the Chaos Brigade.
Aired 01/05/2018EP 4 - The Party of Heroes
2018
High School DxD HERO
Issei and the 2nd years attend a class trip to Kyoto for well-deserved relaxation, leaving the senior and junior class students of the Gremory family behind. However, once there, they fall under attack and must win without Rias or their other powerhouses.
(Source: Anime News Network)
Note: Includes episode 0, Taiikukan-ura no Holy.