Kingdom Hearts Unchained χ (read the "x" as "kee", as it's a Greek letter) is an official, canon Kingdom Hearts game for iOS and Android devices. It was released in Japan in September 2015, in North America in April 2016, in Europe and Australia in June 2016.
The game title refers to the legendary x-blade, a weapon central to the series storyline.
Its presentation is compared to that of a fairy tale, as depicting the usual style of the series would have been difficult on the platform.
Kingdom Hearts Unchained x is the mobile version of Kingdom Hearts x[chi], a Japanese PC Browser game which has been playable from July 2013 to September 2016. Its services have been closed down in order to focus more on Unchained x.
As it was a Japanese exclusive, I worked with KH13.com in the last couple of years in order to provide a full, hand-translated walkthrough of the game.
You can watch it here:
youtube.com/playlist?list=PLhcSrKyqhfh5gWWmRv4rISA…
Bear in mind that Kingdom Hearts Unchained x is not simple mobile transposition of the browser version, but a kind of a different game, which adds to the whole Kingdom Hearts x[chi] experience.
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Kingdom Hearts Union χ[Cross] acts as a 'second season' of Kingdom Hearts Unchained χ which started in March 2017.
Finally, Kingdom Hearts HD 2.8 Final Chapter Prologue, released in January 2017 on PS4, includes the third installment of the whole Kingdom Hearts χ[chi] experience: Kingdom Hearts χ Back Cover, which talks about the foretellers and reveals new parts of the series' history in the form of a one hour cinematic video.