The Shadow Mewtwo card was first revealed on a Nintendo Direct on November 12th, 2015. It will be released with the first production run of Pokkén Tournament in March 18th, 2016.
Pokkén Tournament Edit
The Shadow Mewtwo card allows players to unlock Shadow Mewtwo immediately, without having "to fulfill certain conditions".
Shadow Mewtwo makes its appearance in Pokkén Tournament. Little is known about Shadow Mewtwo at the moment, but surely we will find out what is the story behind Shadow Mewtwo.