The fourth season of High School DxD.

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)

Includes episode 0, Taiikukan-ura no Holy.

 AnimeRulz is the best site to watch High School DxD Hero SUB and DUB in HD quality.