The Shadow Mewtwo card was first revealed on a Nintendo Direct on November 12th, 2015. It was released with the first production run of Pokkén Tournament on March 18th, 2016.
The Shadow Mewtwo card allows players to unlock Shadow Mewtwo immediately without needing to do anything.
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.