Dragon Mountain is a location in the Underworld that the majority of still active Dragons reside in, being lead by the former-Dragon King Tannin after he obtain the rights to the territory upon his promotion to Ultimate-class Devil.
The Dragon mountain is also connected to and a part of the territory of the Gremory Clan. It was given and taken under the care of Tannin alongside his many and various dragons that Tannin took from the Human world. The reason why Tannin took this place was to obtain a certain fruit called Dragon apples in order for his other kin to survive.
In Volume 5, Tannin took the current Red Dragon Emperor; Issei Hyoudou, to the mountains to train in order to increase his dragon power and unlock his Balance Breaker, as the former Dragon King trained Issei by engaging in multiple sparring sessions together.
In Volume 13, upon Issei's hearing request to help train Riser Phenex, Tannin took the two to the mountains and left them to train under the supervision of two of his dragon subordinates.
In Volume 19, it is revealed that Tannin took in Crom Cruach to stay on the mountain as a guest, providing his with food, clothing and shelter. In exchange, Crom Cruach aids Tannin when considered necessary such as transporting the Spectre Dragon egg.
Mount Issei is the mountain where Issei Hyoudou trained at to achieve his Balance Breaker.
- The Dragon Mountain is one of the few places that Dragon Apples grow naturally, which is why Tannin went thorugh great lengths to obtain the territory.
- Mount Issei was named by Rias Gremory as the mountain didn't originally have a name.