According to the Japanese SMTxFE website, Mirage Master form is called “Carnage Form.”
☆ NintendObs Weekly – Monday, October 5, 2015 – Sunday, October 11, 2015.
The World of Genei Ibun Roku #FE:
Glamorous Stage Performance in Modern Tokyo:
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…
…Wanna play? Buy a Wii U.
And if you’ve already got yours, click on Tokyo Mirage Sessions #FE for everything you need to know about the game. 😀