Holiness Behind the Gym
After casting Asia into the Dimensional Gap, Shalba Beelzebub kills Diodora for his failures. Distraught and enraged by Asia's demise, Issei's Juggernaut Drive is activated and he engages and kills Shalba in a duel. Unable to transform back after defeating Shalba due to the incomplete state of his transformation, Issei is stuck in his Juggernaut form. Meanwhile, Vali's team arrives and brings Asia back from the Dimensional Gap, much to Xenovia's happiness. After Vali explains that music has the ability to calm a dragon's rage, Irina appears with a projection device from Odin and it plays the "Oppai Dragon Song".
Aired 10/04/2018EP 1 - Holiness Behind the Gym
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.