Shadow Lugia

Shadow Lugia (XD001) is the only Shadow Pokémon to look different from it's purified form. It is also the end boss of Pokémon XD: Gale of Darkness, the game in which it is named after.

Shadow Lugia
It is the supposed unpurifiable Shadow Pokémon you catch in Pokémon XD when you confront Master Greevil. After snagging or defeating all of Greevil's pokemon, XD001 will appear as the final boss and you will engage it in battle. The easiest way to catch it is to throw the Master Ball, but if you didn't get it from Prof. Krane, you'll need to battle it and use Ultra Balls. It is the most well known Shadow Pokemon.

Purification
According to dialog in the game, XD001 is not able to be purified, however, this is not true. To do this, you must have every chamber in the Purification Chamber working at maximum strength.