|This article is about a/an alternate reality/A.R. World (another rider's world) in Kamen Rider Decade.|
The World of Kamen Rider Black RX (仮面ライダーブラックRXの世界 Kamen Raidā Burakku Āru Ekkusu no Sekai) is the version of reality where Kamen Rider Black RX takes place. The backdrop used to arrive in this world features the Crisis Empire's battleship in the background. Within this A.R. World, Tsukasa originally wore a set of clothes that have him mistaken for Joe the Haze (霞のジョー Kasumi no Jō) by Kotaro and the Crisis Empire. Daiki's treasure in this world is Apollo Geist's Perfecter.
Tsukasa found himself wearing strange clothes with another strange card as he, Natsumi, and Yusuke found themselves attacked by the Strange Demon Robot Schwarian and his Chaps as Kamen Rider Black RX battled them, with both sides believing Tsukasa to be Joe the Haze. When the Seamoon Fangire and Scorpion Imagin suddenly appeared, Tsukasa became Decade to help RX out against the monsters with Agito's power as Schwarian rans off. After Natsumi stopped RX from attacking Decade, RX took her words to heart and saw Tsukasa as an ally. From there, he introduced himself as Kotaro Minami and said that had been fighting the Crisis Empire. Everyone was confused at the sudden appearance of Imagin and Fangires in this A.R. World with Tsukasa reluctant to fight, once more. While Tsukasa took pictures, after Natsumi offered him to embrace her world as his home, they confronted a man named Guy who was behind the attacks. Introducing himself as Dai-Shocker's Apollo Geist, he knocked the Decadriver out of Tsukasa's hand as Natsumi took an attack meant for him as the villain and his attendants left. Natsumi was hospitalized beyond medical aid and Tsukasa was upset by Narutaki telling him that he is a danger to himself and those around him. Daiki visited Tsukasa and Natsumi, telling Tsukasa about Apollo Geist being from the World of X-Rider and possessing the Perfecter, which is the key to returning Natsumi to normal. Tsukasa gone off to confront Apollo Geist on his own, not wanting anyone else to get hurt, finding Schwarian as Apollo Geist attacked him for refusing to deliver his offer to Emperor Crisis to join Dai-Shocker. With RX's support, Decade battled Apollo Geist before Diend interfered with his own Riders to steal the Perfecter. When Apollo Geist summoned the Rhino Mutant, he convinced Schwarian to accept his offer of joining his group as Decade assumed Complete Form to use Kabuto Hyper Form's power to take out the summoned Riders. Apollo Geist escaped into another dimension with Decade in pursuit as RX held off the monsters. Arriving at the other end, Decade finds Kotaro Minami who transformed into Kamen Rider Black to attack him.
Following Apollo Geist into another A.R. World, Decade was attacked by Kamen Rider Black until Diend arrived and summoned Femme to hold him off. The next day, after learning that he was in the World of Black and seeing that he was on his own in saving Natsumi, Decade ambushed Guy as Ryuki and fought him until he used a little girl as his hostage. When Decade took a hit from Diend to save the child, Black watched and had second thoughts about him as Apollo Geist escaped. After explaining that Apollo Geist came to the world before and recruited the cult Gorgom into Dai-Shocker, Kotaro revealed that Joe the Haze ended up in his world and had been helping him fend off Dai-Shocker's endless army. Schwarian arrived and sent the Scorpion Imagin and Mantis Fangire after Decade and Black. Using Faiz's power, Decade destroyed the two monsters as Schwarian summoned the Chaps, allowing Tsukasa to make it back to World of Black RX as Apollo Geist summoned a pack of Orphnoch and Worms. The numbers evened out as Daiki arrived and used Decade's card to summon the World of Black's Kotaro. With Black and Black RX fighting with them, Decade and Diend battled the monsters as Decade assumed Complete Form and used Agito Shining Form's power to destroy Schwarian. After getting the Perfecter from Apollo Geist, Diend stole it as Yusuke arrived to tell Tsukasa that Natsumi was about to die. Once there, Daiki shown up to give the Perfecter to Tsukasa, who used a bit of his life force to revive Natsumi and bring her back to life. Black and Black RX continued their battle Apollo Geist until he is escaped into another A.R. World with Narutaki comments that Decade was to blame for Dai-Shocker. After checking up on Kotaro and telling him that Joe was aiding his other self, Tsukasa returned to the Hikari Studio where he and his friends ventured into the next world.
As one of the New Worlds, what happened to the World of Black RX during the convergence of the Nine Worlds and the Rider War is unknown. However, it can be assumed that, whatever was its fate, all was well after the destruction (and subsequent revival) of Decade ended the Rider War and the worlds were restored.
- Kotaro Minami (Kamen Rider Black RX)
- Crisis Empire
- Dai-Shocker members in the World of Black RX