Shadow Pokémon are Pokémon whose hearts have been closed. They were originally created by Cipher, in Pokémon Colloseum and Pokémon XD: Gale of Darkness. However, in recent years, they have been revived by Team Plasma, in Pokémon XD 2.
Shadow Pokémon act very aggressively. They use unique moves, such as Shadow Rush, to attack other Pokémon. Shadow-based attacks are super effective to non-Shadow Pokémon, but are not very effective against Shadow Pokémon. They are aggressive enough to even attack people!
In Fan Games
Shadow Pokémon make an unruly return, and Team Plasma plans to use them to take over the entire region of Unova!