|This article is about a/an alternate reality/A.R. World (another rider's world) in Kamen Rider Decade.|
The World of Diend (ディエンドの世界 Diendo no Sekai) is the world where Daiki comes from, similar to the World of Blade due to the presence of Undead known as Roaches (ローチ Rōchi). More specifically, it seems to be related to the film Missing Ace, due to being home to the three Ace Riders (even played by the same actors) and Jashin 14 (known as the Fourteen), characters exclusive to the film. But as the world is dominated by the Roaches and their leader Fourteen, the human populace is mostly brainwashed into a state of euphoria and conditioned to see Kamen Riders as disruptors of the peace, and anyone who disobeys the rules, even slightly, has their free will completely removed. The backdrop used to enter this world depicts a wanted poster with Diend's symbol on it in the foreground and a cityscape in the background. Within this A.R. World, Tsukasa assumes the role of a traveling salesman before being labeled a criminal. Daiki's treasure in his homeworld is freeing his brother from the control of Fourteen, only to learn he was not controlled at all.
Arriving in Kaito's homeworld, the World of Diend, Tsukasa found the world in peace and harmony, but with Kaito a wanted man. As Tsukasa assumed the role of a traveling salesman, he met Kamen Riders Larc & Lance as they battled a Darkroach. After destroying the monster, Tsukasa learned that the world's residents kill Kamen Riders. As Tsukasa and company met the head of area management committee, Junichi Kaito, Larc and Lance battled the Darkroaches until they received aid from Diend. Saving them from reinforcements, Tsukasa learned from Haruka and Shin that the world was ruled by the mysterious Fourteen who uses his Roaches to manipulate humans. Cornered by the Bossroach and his Darkroaches, the Kamen Riders battled the Roaches before Lance gone after Diend. While this occured, Yusuke is kidnapped by the Roaches to be conditioned into being a normal member of the World of Diend's society.
Nearly killed by Lance, Kaito was spared when Larc stopped her partner as he reminded her that he was the reason that Junichi was under Fourteen's control. After escaping the angry mob, parting from Haruka and Shin, Kaito and Tsukasa found Natsumi who told them that Yusuke had been kidnapped by the Roaches before Junichi arrived. After remembering the events leading to his brother's enslavement, Kaito was reluctant to fight as he and Tsukasa transformed to follow Junichi, leaving the brain-washed Yusuke behind and taking Natsumi to undergo the process as well. The next day, finding himself a wanted man as well, Tsukasa decided to help Kaito in spite of him refusing his aid at first. Using Kaito as bait to trade for the now brain-washed Natsumi, Tsukasa lured out Fourteen. While Lance and Larc battled Glaive and the Darkroaches, Diend and Decade battled Fourteen as he revealed his true form as Jashin 14. Though overwhelmed, Decade assumed Complete Form and uses Armed Hibiki's power to destroy him. Though the Roaches faded away and the brainwashed humans returned to normal, Glaive attacked Diend and revealed that he willingly served Fourteen. With Decade holding Larc and Lance back, Diend battled Glaive on his own in a heated battle with neither wanting to kill the other. Tsukasa ended the battle, telling the brothers that Daiki now had faith in himself. Junichi left and Tsukasa and Kaito gone off to the next world, a strange one that Narutaki hoped would overwhelm Decade.