Underworld

The Underworld (冥界 Mekai) is a location in High School DxD, and the home of the Devils and Fallen Angels.

Appearance
The Underworld is roughly the same size as the human world but with a much larger landmass, as there are no oceans. The sky is originally purple during both the day and night but the current Four Great Satans adjusted the flow of time to match it in the human world for the sake of the reincarnated Devils. They reproduced a false moon using magic and made the night's darkness the same with the human world.

The Underworld was originally split into two regions: The Devils' Region and the Fallen Angels' Region, which ended in Volume 4 after the creation of the peace treaty between the leaders of the Three Factions.

Lilith
The current Capital of the Satan territory.

Lucifaad
The original Capital of the Satan territory.

Agreas
A large city that is located in the Agares Territory which is being maintained by the servants of the current Beelzebub. It has a distinctive feature in which it appears to be a large floating island suspended in the sky. A popular tourist resort, it is also the place that controls the flow of the air in the Underworld. The Agreas is a place that has the technology of the Original Satans and contains the essential crystals of the highest quality that is used to create both the Evil Pieces and Brave Saint. In Volume 17, Agreas was taken by Qlippoth. It was finally located by the Vali team in volume 20, where Qlippoth had been using the city to both wake Trihexa and create mass-produced Boosted Gear Scale Mails.

Auros
An agricultural town located in the Agares Territory which is said to have the best agricultural products in the Underworld. Sona Sitri's Rating Game school is being built here.

Gremory Territory
The Gremory Clan has a territory that is about the same area of Honshu in Japan. A very rich territory with developed industry. Among the territory is the Gremory Palace the home and main building of the Gremory Clan Head.

Realm of Dead
Also known as the realm of Hades in Greek mythology.The place where Hades the Greek god of the dead and the Grim Reapers reside. Located in the lower strata of the Underworld and the place where the souls of the dead are sorted and also close to the Cocytus. The realm of the dead is of the Olympus World the God from the Greek-faction Hades is in control of it. It isn't as big as the Underworld and it's the World of the dead, it is a wasteland that no living creature can live in. It also has an ancient Greek shrine as it appears into the depths. It is the place where the Grim Reapers of this World live as well as the castle of Hades.

Purgatory
Purgatory was prepared for the deceased with special circumstances and to purify the spirits and souls who have died. It also the place were those who have died have been judging if they were going to Heaven or Hell. It was also connected in one of the doors of Heaven like Third Heaven. This is the place were Cao Cao use to enter Heaven in the Third Heaven.

Limbo
The edge of the Underworld, this is also the place were wicked souls and spirits remains and also the burning place where those wicked souls and spirits punish them for their sins. It was also connected in one of the doors of Heaven.

Cocytus
The deepest part of the Underworld that is connected to the Realm of the Dead. It acts as a prison for powerful and dangerous figures such as Samael and Kokabiel. It is also the home of many rare and dangerous creatures such as the Pale Horse.

Trivia

 * The current capital of the Satan territory, Lilith, is a reference to the female demon Lilith from Christian and Hebrew beliefs, mother of monsters.
 * Seemingly named for Lucifer's wife, Rizevim's mother.
 * There is a hamburger restaurant chain in the Underworld named Akuma-naldo.