|This article is about a/an event in Kamen Rider Decade.|
- For the great battle from Kamen Rider Ryuki, see Rider War (Ryuki). For the episodes of Kamen Rider Decade that feature this war, see Rider War (episode) and Rider War: Prologue. Also not to be confused with the Rider Battle tournament seen in Kamen Rider Decade: All Riders vs. Dai-Shocker.
After Tsukasa Kadoya traveled to multiple A.R. Worlds, Wataru Kurenai appeared before him saying that was true mission to defeat the Nine Heisei Riders. Wataru stated that in order to save the worlds, Decade should have defeated those worlds Riders in order for these worlds' stories to unite with each other through him. Instead, by befriending the Riders of each world, Tsukasa unintentionally did not prevent the merging. The result was that all the A.R. Worlds' Riders faded (with the exception of Yuusuke Onodera).
After being attacked by the original Riders, Tsukasa realized what needs to be done, thus allowing Decade's base form to evolve. Known as Decade Violent emotion, this form allowed him access to every Kamen Rider powers without transforming to its respective Rider. With this new power, Decade became more aggressive. One by one, he hunted the Riders in order to fulfill his true mission, separating himself from the others.
After defeating the pre-Decade Riders, Tsukasa allowed Kamen Rider Kiva-la to mortally wound him, so that Natsumi can become the connection between the riders and the Rider War would end in Decade's death.
- All riders that existed from Kuuga to Decade.
- The Rider War of Decade and the Legend War of Kaizoku Sentai Gokaiger, which saw the first 34 Super Sentai, the predecessors of the Gokaigers, joining forces and sacrificing their powers (which resurfaced as Ranger Keys) to destroy the invasion armada of the Space Empire Zangyack, were noted by Narutaki as one of the similarities between Decade and Gokaiger in the Super Hero Taihen net movie The Grown-ups’ Narumi Detective Agency Ep. 1: Is Gokaiger a Ripoff of Decade?.