|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 (仮面ライダーブラックの世界 Kamen Raidā Burakku no Sekai) is an alternate reality where a version of Kamen Rider Black takes place, a parallel world to the World of Black RX. Apollo Geist arrives in this world prior to its RX counterpart, recruiting the cult Gorgom as a result.
During a fight in the World of Black RX, Apollo Geist of Dai-Shocker escaped into another dimension with Decade in pursuit. Arriving at the other end, Decade found a Kotaro Minami who transformed into Kamen Rider Black to attack him 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 escaped into another A.R. World with Narutaki commenting 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 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.