For the uninitiated, Kamen Rider Diend first appeared in the Kamen Rider Decade TV series. Unlike Kamen Rider Decade who has the ability to transform into past Kamen Riders, Kamen Rider Diend has the ability to summon past Riders thanks to the Diend Driver. The last time we saw him in the Kamen Rider franchise was in 2012's Kamen Rider × Super Sentai: Super Hero Taisen film.
[Source: SpecialForm12]