Shadow Pokémon are additions to the game that were first seen in Pokémon Colosseum. They are normal Pokémon that have had their hearts closed off by Cipher, a dark team that wreaks havoc all around the Orre region.
Shadow Pokémon have a dark aura lurking around them, which causes them to be identified as Shadow without examination. Only one person is able to see that, and that is your partner in Pokémon Colosseum. The protagonist from Pokémon XD: Gale of Darkness uses his Snag Machine, which has an attached scanner, to see the Shadow Pokémon.
The Shadow Pokémon are revealed to have a fierce, hateful personality. They will attack trainers at times, and at other times they will fulfill their duty as a Pokémon Battler. Most of the Shadow Pokémon from Pokémon XD: Gale of Darkness have a powerful attack that causes recoil.
 In Battle
Instead of the traditional "Fight", "Items", "PKMN" and "Run", Colosseum replaces "Run" with "Call". Call helps you open the heart gauge of a Shadow Pokémon, which speeds up the Purification Process.
All Shadow Pokémon have one Shadow attack, titled "Shadow Rush". As you purify them, they will learn more attacks, but these are attacks that the Pokémon knew before their heart was sealed.
 Gale of Darkness
Instead of the traditional "Fight", "Items", "PKMN" and "Run", XD replaces "Run" with "Call". Call helps you open the heart gauge of a Shadow Pokémon, which speeds up the Purification Process.
All Shadow Pokémon have at least one Shadow attack, but some can have up to 4. These include new Shadow attacks, such as "Shadow Sleep" and "Shadow Stop". These attacks are like normal attacks. Shadow Sleep serves the same purpose as Spore and Hypnosis, causing the Pokémon that get hit by it to go to sleep. While you are freeing their heart, the Pokémon regain old moves if all 4 Move Slots are not taken up.
All Shadow Pokémon have a Shadow Gauge that slowly goes down when you are purifying your Pokémon. There are many ways to get the gauge down. These include:
The final process of Purification is taking it to the Relic Stone in Agate Village. If you take your Shadow Pokémon there, and it's Shadow Gauge is depleted, it will be purified, gaining all the Exp. that it has earned in battle, and obtaining all of it's old moves. It will now gain Exp. at a normal rate, and will lose it's Shadow Attacks.
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