Demon Realm

The Demon Realm or Demon World (魔界 Makai) also called Netherworld and Dark Side is an alternate dimension where the demons ruled by the Dark Lord reside. It is able to corrupt the souls contained within it and is also where Dracula's soul is sent to whenever he is destroyed.

The Demon Realm was first mentioned to exist on Castlevania III: Dracula's Curse's japanese manual. There it's said that Dracula could summon demons from this place with the assistance of false gods. The japanese manual for Castlevania: The Adventure also mentions an "other world" where Dracula summoned his demons from, this time by himself and already being the Demon King. It's likely that this "other world" is the same Demon Realm mentioned on the Castlevania III's japanese manual.

The Demon Realm was dubbed "Netherworld" in the english localization of Symphony of the Night under Galamoth's description. There it is revealed that Galamoth seeks to take over the throne of this dimension in a 10,000 years plan. Interestingly, Galamoth does succeed on his plan on "Boku Dracula Kun!" but is ultimately defeated by Kid Dracula.

On the english localization for Dawn of Sorrow this place is referred to as the "Dark Side". Celia Fortner has the power to open gates to the Demon Realm and, with this ability, she was able to make her demons invincible unless Soma can use Magic Seals. By opening a portal to the Demon Realm she plans to turn her castle into a Demon Castle such as Dracula's Castle. She also says that the power of the Demon Realm is growing weaker and that a new Dark Lord must be crowned post haste, implying that the lack of a Dark Lord may be the cause for the darkness' weakening.