From Bulbapedia, the community-driven Pokémon encyclopedia.
Citadark Isle is the headquarters of Cipher in Pokémon XD. It can only be reached by riding the Robo Kyogre from Gateon Port. A large number of Shadow Pokémon are found on Citadark Island, and the player will defeat Lovrina, Snattle, and Gorigan, and also fight Ardos and Eldes. After defeating all of Cipher's admins, the player will battle XD001, the Shadow Lugia that is supposedly impossible to purify.
Once XD001 has been captured (recommended, because this is the only way to get Lugia in any of the Generation III games without going to an event) or defeated, Greevil will proceed to battle the player, using a team of only Shadow Pokémon. After Greevil is defeated, Ardos will come in and tell Greevil to blow up Citadark Isle. However, Eldes will also come in and convince Greevil that doing this would be immoral. Greevil realizes that everything they have done was a mistake, and he and Eldes turn themselves in to the police.