|This article is about a/an alternate reality/A.R. World (another rider's world) in Kamen Rider Decade.|
The World of Ryuki (龍騎の世界 Ryūki no Sekai) is the alternate reality where a version of Kamen Rider Ryuki takes place. In this world, there exists the Kamen Rider Trial (仮面ライダー裁判 Kamen Raidā Saiban), a battle where the Kamen Riders fight each other within a parallel dimension known as the Mirror World (ミラーワールド Mirā Wārudo) alongside their Contract Monsters (契約モンスター Keiyaku Monsutā) in order for the victor to decide the fate of a defendant in a criminal case. Though no harm occurs to the Rider while in the Mirror World, a battle in the real world can be fatal and use of Vent cards outside a battle is forbidden. The backdrop used to arrive in this world shows Dragreder flying within the Mirror World as shown in a mirrored time display screen on a skyscraper. Within this A.R. World, Tsukasa assumes the identity of a lawyer and the ability to enter the Mirror World without an Advent Deck.
Upon arriving in the World of Ryuki, Natsumi gone to the Atashi Journal headquarters to learn about the world's Kamen Riders from its editor, Reiko Momoi. After Natsumi's arrival at the journal's offices, Reiko was mysteriously murdered, leaving Natsumi as the culprit as she had a small fork in her hand. Natsumi was arrested under suspicion of murder, and was to be sentenced with a Kamen Rider Trial: a system where Kamen Riders fought in the Mirror World to determine the outcome of criminal cases. Natsumi called upon Tsukasa to be her lawyer, who met with Shinji Tatsumi to hear what he knew of what happened to Reiko. As Shinji revealed that he is Kamen Rider Ryuki and he believed Natsumi was framed, his former friend Ren Haguro (Kamen Rider Knight) was fighting in the Kamen Rider Trial, defeating several opponents. Shinji suggested to Tsukasa that he should fight as Decade in the Kamen Rider Trial to clear Natsumi's name. At the scene, he found that Ren Haguro was also at the scene of the crime before he was brought into the Mirror World along with the sub-editor of the Atashi Journal: Kamata (Kamen Rider Abyss). Decade and Abyss' fight, however, was interrupted by Kamen Rider Impaler, and Abyss left to watch Decade's abilities. Elsewhere, Shinji confronted Ren, believing he was Reiko's real murderer, and he challenged him as Kamen Rider Ryuki. Their fight was also interrupted by Decade, having just defeated Impaler and now turning his sights on Knight, easily overwhelming him in Kiva's various forms with Shinji watching.
After defeating Knight, Decade was about to finish him off when Ryuki pleaded with him not to deliver the final blow, as he felt Knight must answer for his crime by his hand. However, Abyss used this time to make his move to take out all three Riders at once, forcing them to retreat with Ren refusing to answer Shinji as he left. After her brief encounter with Narutaki, Natsumi learned that Shinji and Tsukasa were convinced that Ren was Reiko's murderer, although Yusuke saw otherwise. Meanwhile, a wounded Ren battled Kamen Rider Odin and defeated him, emerging from Mirror World with Odin's Time Vent as Shinji and Yusuke found him. Learning that Ren obtained the card to save Reiko, Shinji used the card to go back in time as Tsukasa held Abyss off. Arriving seconds before Reiko's death, Shinji and Tsukasa managed to save Reiko with the latter cornering her true murderer, Kamata. Despite this, Kamata arrived and absorbed his past self before becoming Abyss and unleashing his monsters on Decade. After learning the truth behind Ren coming to the Atashi Journal and why he left, a relieved Shinji ran to Tsukasa's aid. Though the two managed to defeat Abyss together, they learned from Narutaki that Kamata was actually an Undead as the two departed. After he received Shinji's photo of himself and Yusuke, Tsukasa and company entered the World of Blade.
Shinji later appears in Kamen Rider × Kamen Rider W & Decade: Movie War 2010 alongside the other Riders of the Nine Worlds to help in reviving Tsukasa and helps him battle Super Shocker, assuming Ryuki Survive with the help of Decade's K-Touch and later into his Ryuki Dragredder form with the All Rider Final Form Ride Card to fight the Super Crisis Fortress.
- Shinji Tatsumi
- Ren Haguro
- Reiko Momoi
- Other Kamen Riders
- Mirror Monsters
- Contract Monsters
In the novel reimagination of the Kamen Rider Decade TV series, Kamen Rider Decade: The World of Tsukasa Kadoya ~The Garden Inside the Lens~, the World of Ryuki is, as one of the Nine Worlds, not an alternate reality like in the TV series but the original world. The World of Ryuki is one of the six worlds the Hikari Studio group, Tsukasa/Decade, Kaito/Diend, and Natsumi, have already traveled to by the start of the novel as they head for the three remaining worlds, and thus not much is seen about it.