The Demon Realm
Translation: demon world, demon realm
AKA: 悪魔界/akumakai (“Devil Realm”), 暗黒魔界/ankoku-makai (“Demon Realm of Darkness”)
Other Names: the Underworld, the Netherworld, the Spirit World (all various fansubs)
The Demon Realm is the lower half of the living world, a dimension that exists on the reverse side of the universe. It’s not to be confused with Hell, which exists in the afterlife. According to Daizenshuu 7, it’s a chaotic place where magic has more influence than science. The Kaio/shin themselves don’t really understand the Demon Realm, as not even the gods’ eyes can reach it. As a result, not much is known about it other then that it is the home of evil life-forms. Records still survive of these life-forms interfering with the peace of the universe’s planets. In fact, one of the factors in the nations of the DB Earth unifying into a single world state was to guard against this threat from the Demon Realm.
Like the universe with its division between the four cardinal directions, the Demon Realm is divided into several different spatial areas, though it’s unknown how many there are or what these divisions are based on. Either way, all these areas are unified under the rule of Dabra, the king of the Demon Realm and far and away its strongest resident, according to Kaioshin. Or at least they were, until Dabra was brainwashed by the wizard Babidi into becoming his servant. Dabra’s been working for Babidi for at least the last 300 years, and it’s not known who if anyone has taken control of the Demon Realm in Dabra’s absence.
According to Toriyama in the recent Super Exciting Guides, the Demon Realm has its own special gods called Makaio and Makaioshin (“Kings of the Demon Realm” and “Gods of the Kings of the Demon Realm”, respectively). They’re the antitheses of the Kaio and Kaioshin, and govern evil. Whenever one of the Shinjin (the special race that Kaio and Kaioshin are selected from) is born with an evil heart, they leave the Shinjin’s planet and go to be with the Makaio. However, it’s unknown whether the Makaio/shin themselves are selected from among these delinquent Shinjin, or whether they are beings who have always existed in the Demon Realm and simply ally themselves with evil Shinjin. It’s also not known whether Dabra himself was a Makaio/shin, or if he was simply a political ruler of the Demon Realm rather than a god (like the difference between the Earth’s World King and its God). At any rate, Toriyama says that currently the power of the Kaioshin is greater than that of the Makaioshin.
In the manga we never officially see the Demon Realm, but when Gohan and Dabra are fighting Babidi teleports them to some different world, one of rocky plains and blood-red water. Since Babidi transported his other henchmen Pui Pui and Yakon to their respective home worlds during their fights with Goku and co., it seems likely that this is the Demon Realm. Also, when Dabra dies Enma Daio sends him to Heaven on the reasoning that he’d be happy to go to Hell. This implies that Hell and the Demon Realm are similar, and in fact the place where Gohan and Dabra fight does somewhat resemble Hell as seen in the anime. Also, the two realms are positioned similarly in their respective worlds: Hell at the bottom of the afterlife, and the Demon Realm at the bottom of the living world.
The Demon Realm also figures into the anime filler showing Goku’s journeys while training for the 22nd Tenkaichi Budoukai, made years before the Demon Realm was ever mentioned in the manga (though “Demon World” is a fairly generic term that pops up in lots of manga and anime, so it’s use here is probably a coincidence). In DB episode 81, Goku visits a village being attacked nightly by beings from the Demon Realm. In another premonition of things to come, the inhabitants of the Demon Realm are called majin in this episode, with females being called majo, literally “demon woman” but generally used to mean “witch”. These fiends are coming from the Demon Realm Gate, a portal linking the living world to the afterlife, and located in a cave connected with the Demon Realm. This portal is only supposed to be used by grim reapers, who are authorized to do so by Enma Daio and God (mentioned in yet another bit of unintentional foreshadowing).
A Demon Realm martial artist named Shura jams a cursed sword into the gate to keep it open, allowing him and his fellow fiends to come and go freely between the Earth and Demon Realm. Eventually Goku manages to set all this right, mostly by grabbing a princess Shura had kidnapped and running away very fast. This filler version of the Demon Realm has several inconsistencies between the Demon Realm that later figures in the Boo arc (such as the Demon Realm apparently being part of the afterlife), but it’s still a neat little side story.