The Japanese website has been updated with a bunch of new screenshots and details about the game's Mirage mechanic.
There's an online mode called The Arena where you can battle Mirages with other players online.
For capturing a Mirage, you must meet the requirements for each. There are special conditions for these like using a specific item and once that condition is reached the Mirage will glow. You can name each Mirage that you befriend and you can teach it new abilities and increase its stats, but it will cost you Skill Points which you'll gain by leveling up your Mirages. Mirages in your reserve party will also acquire experience points.
There is a Mirage Board where there are blocks for transforming Mirages. Transformations make them more powerful and all abilities carry over when you transform except for exclusive abilities. There is also the option to reverse the transformation if you so choose.
The protagonists of this game originate from a place called Nine Woodshill and the player will travel from there to the world of Grymoire, that seems to be a world that entwines the many worlds of Final Fantasy.
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