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靈魂被審判後會進入冥界：所有持罪的靈魂將會受到懲罰，被關在冥界的監獄內，懺悔自己的罪過；另一方面，善良的靈魂可以自由待在出生於冥界本地的惡魔 Haros 族一起生活，等待著轉世的日子。
幽冥列車301是將死者的靈魂運送冥界的渡輪。它承載著乘客、囚犯、自衛隊、女服務員和一位列車長。However, the train is also proven to be sentient enough to engage in an automatic defense system upon its conductor's defeat.
There are portals or gateways connecting Aernas to the Underworld, a result of the weakening of the dimensional barriers separating the two worlds, which in turn is due to the breaking of the seal between Aernas and Elyos, the land of the Asmodians. The few known portals are the one from which the Nightmare Circus escaped through, and the one Rufus used to pursue refugee souls in Aernas. Berkas is also known to have came into Aernas through a portal. The Sanctum of Destruction and The Crucible also originate from this world. It is likely that the Hell Bridge in the Ellia Continent houses or is one of these gateways.
It is also likely that there are portals connecting the Underworld to Elyos. The Asmodians know of the Haros, and even they fear the bounty hunters as evidenced by Dio, who with his immense powers was overwhelmed by Rufus's speed and chose to flee from him when the Haros targeted Ley in the Wizard's Labyrinth.
The Haros are a native tribe of demons residing within the Underworld and are the guardians of that world. Amongst them are the Bounty Hunters, warriors held responsible for chasing down not only the evil souls that might escape from the Underworld's prison, but also any creatures native to the Underworld that have fled into Aernas. Bounty Hunters have special abilities that allow them to either send the refugee back to the Underworld, or exorcise them completely. However, they usually do not pursue creatures that do not belong in the Underworld.
Though the Haros and the Asmodians are both demons, the main difference between them is the lack of horns, paler skin tone, and shorter ears for the Haros as compared to the Asmodians. Haros half-breeds are identified by their hideous appearances, such as scale-shaped growths on their faces.
Despite its status as the world of the dead, the Underworld ironically hosts a number of races that are living as well.
The Underworld is also home to the Iron Dragon species from which Berkas hails. The power structure within the Iron Dragon species has been firmly established for thousands of years, with the younger dragons as subordinates to the ancient and more powerful dragons. Berkas, who is unsatisfied with his place within the power structure of his species, chooses to escape into Aernas where the power flow is less stable.
The service crew and conductors of the express service that ferries dead souls into the underworld, such as those of the Monster Train 301, are residents of the Underworld as well. The only known conductor so far is Casey Bones, who has lead the Monster Train 301 for centuries. However, after his defeat, the train engages an automatic defense system and swallows him, though Jin claims that he can still feel the conductor's aura.
- The Underworld is also known as Hades or Arallu.
- The floating islands in the Underworld's background resemble an upside down image of the floating landmark in the background of Hell Bridge.
- Though Lass is a half-Haros, it is unknown how he relates to the rest of his tribe as he is the leader of the Silver Cross Thieves in Aernas. It is also very unlikely that he has ventured to the Underworld prior to the events of the Monster Train 301.