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You have to get a 50 win streak in the battle tree three times, then defeat the elite four with a team of 6 shiny level 100 magikarp. You will then be challenged to a double battle by Mario and Luigi. After defeating them they will give you the "Shadow Idol.". This item will transfer over to your next save. You then need to start a new game and you will be given the option to choose Marshadow as your starter. Have fun with your new Marshadow OP!
