The Nightmare Stone will allow your Ponymon to evolve into their Nightmare Night form, if applicable. To use it, press the Z button on the stone, and select use. If your Ponymon is displaying the word 'ABLE', the Nightmare Stone is applicable with that Ponymon. When the Nightmare Stone is used on a Ponymon that it is applicable with, the Ponymon will evolve into it's Nightmare Night form. For example, Pinkie Pie will become NPinkiePie when exposed to a Nightmare Stone. Any Ponymon in it's Nightmare Night form will have dark type in them, except for N Derpy.