The Dark Realm (Phantom Realm) is an underworld arena where the player fights through stages of genma. This sidequest has featured throughout the main series and is often accessed through Minogoro and his son, Minokichi – a member of the Mino Clan, who possesses the power to teleport between worlds. There is a document in Onimusha 3: Demon Siege, which refers to the Dark Realm as a 'training ground' where Oni warriors can fight against the imprisoned genma.
In the very depths of this world, Onimusha warriors can find powerful weapons and items that have been sealed away to stop them falling into the wrong hands. In Onimusha: Warlords, these weapons and items were the Bishamon Ocarina and Bishamon Sword. This location has appeared in Onimusha: Warlords, Onimusha: Samurai's Destiny, Onimusha: Demon Siege, and Onimusha: Dawn of Dreams.