Rider War (Decade)

The Rider War (ライダー大戦) is a cataclysmic event which occurred during the merging of the A.R. Worlds which was by then called the World of the Rider War.

After Tsukasa Kadoya traveled to multiple A.R. Worlds, Wataru Kurenai appeared before him saying that actually a test was 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 Yusuke 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 goal, 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.