|This article is about a/an episode in Kamen Rider Decade.|
The Nega-World's Dark Riders (ネガ世界の闇ライダー Nega Sekai no Yami Raidā) is the twentieth episode of Kamen Rider Decade. It is the first part of the World of Negatives arc, the first of the New Worlds the Hikari Studio travels to following the journey across the Nine Worlds.
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With their tour of the nine Kamen Rider Worlds complete, the Hikari Studio group returns to their home world, finding it as it was before the monsters appeared. Tsukasa is the only one who feels something is not right and questions the reason for the journey. Before Yusuke or Natsumi can attempt to answer, they encounter the violinist Otoya Kurenai, who thanks them for saving the world and tells Tsukasa that he earned a world of his own for his journey. As Natsumi goes out to see her TG Club friends and teacher for a high school reunion, Tsukasa and Yusuke have a short encounter with Narutaki who congratulates him on getting his own world. Later, Tsukasa becomes the ten-thousandth customer in a restaurant and wins both a free meal as well as a vast fortune. Tsukasa enjoys the upper class lifestyle until he takes his leave during an arranged marriage meeting. Yusuke is forced to take his place, but he runs off upon realizing that the women are actually monsters in disguise. Elsewhere, while hanging out with her TG Club friends, Natsumi tries to figure out where her friend Chinatsu is, and the TG Club members take the opportunity to run off, with the teacher attacking a couple as Alternative. Tsukasa runs into Otoya who mentions a treasure before transforming into Dark Kiva and challenging him, along with the other members of the TG Club who become Ryuga, Dark Kabuto, and Orga. With Diend arriving to back him up for this treasure, Decade battles Dark Kiva. When Decade attempts to use his Final Attack Ride against Dark Kiva, the attack suddently cancels as he falls and reverts back to human. As the dark Riders leave, Tsukasa realizes that all of his Rider cards have turned black. Elsewhere, Natsumi is still searching for Chinatsu when she passes by someone in dirty and ragged clothing who turns out to be another Natsumi
- Tsukasa Kadoya (門矢 士 Kadoya Tsukasa): Masahiro Inoue (井上 正大 Inoue Masahiro)
- Daiki Kaito (海東 大樹 Kaitō Daiki): Kimito Totani (戸谷 公人 Totani Kimito)
- Yusuke Onodera (小野寺 ユウスケ Onodera Yūsuke): Ryouta Murai (村井 良大 Murai Ryōta)
- Natsumi Hikari, World of Negatives' Natsumi (光 夏海, ネガの世界の夏海 Hikari Natsumi, Nega no Sekai no Natsumi): Kanna Mori (森 カンナ Mori Kanna)
- Eijiro Hikari (光 栄次郎 Hikari Eijirō): Renji Ishibashi (石橋 蓮司 Ishibashi Renji)
- Narutaki (鳴滝): Tatsuhito Okuda (奥田 達士 Okuda Tatsuhito)
- Kiva-la (キバーラ Kibāra, Voice): Miyuki Sawashiro (沢城 みゆき Sawashiro Miyuki)
- Decadriver Voice, Diendriver Voice (ディケイドライバー音声、ディエンドライバー音声 Dikeidoraibā Onsei, Diendoraibā Onsei): Mark Okita (マーク・大喜多 Māku Ōkita)
- Narration (ナレーション Narēshon): Eiichiro Suzuki (鈴木 英一郎 Suzuki Eiichirō)
- Otoya Kurenai (紅 音也 Kurenai Otoya): Kouhei Takeda (武田 航平 Takeda Kōhei)
- Hirohiko Sato (佐藤 博彦 Satō Hirohiko): Takatsugu Iwama (岩間 天嗣 Iwama Takatsugu)
- Kenji Sakata (坂田 健児 Sakata Kenji): Keisuke Sakamoto (坂本 恵介 Sakamoto Keisuke)
- Kazuyoshi Aoyagi (青柳 和良 Aoyagi Kazuyoshi): Yohta Mori (森 陽太 Mori Yōta)
- Tanaka (田中 Tanaka): Masayasu Kitayama (北山 雅康 Kitayama Masayasu)
- Chinatsu (千夏): Juri Ihata (井端 珠里 Ihata Juri)
- Restaurant Owner: Kouichi Oohori (大堀 こういち Ōhori Kōichi)
- Maid: Shiori Maeda (前田 紫織 Maeda Shiori)
- Kamen Rider Decade: Seiji Takaiwa (高岩 成二 Takaiwa Seiji)
- Kamen Rider Diend, Kamen Rider Dark Kiva : Eitoku (永徳)
- Kamen Rider Ryuga: Jiro Okamoto (岡元 次郎 Okamoto Jirō)
- Card Used:
- Kamen Ride: Decade, Ryuki
- Attack Ride: N/A
- Form Ride: N/A
- Final Form Ride: N/A
- Final Attack Ride: Decade
- Decade, Decade Ryuki
- Kamen Ride: Diend
- Attack Ride: N/A
- Final Form Ride: N/A
- Final Attack Ride: N/A
- Hirohiko/Ryuga's transformation sound reuses the casual sound rather than Ryuga's own.
- Kazuyoshi/Dark Kabuto and Kenji/Orga's Transformation devices lack any of their respective voice lines.
- As part of Super Hero Time, this episode aired alongside Samurai Sentai Shinkenger episode 17, Act 17: The Sushi Samurai.
- Viewership: 7.9%
Blu-ray Box comes with all 31 episodes.