Dark Lugia is a rare Pokemon revered for its unmatched shadow power and deadly attacks. It is native to "Pokemon XD: Gale of Darkness," but can be caught in "Pokemon SoulSilver" through the use of a combination of Action Replay cartridge cheat codes. "Dark Lugia" still will have to be encountered and fought in battle, but by following a few simple steps you can catch this legendary powerhouse for yourself.
Insert the Action Replay cartridge into the top of your Nintendo DS console and turn on the console.
Select the player profile in which you want to catch a "Dark Lugia."
Enter a "very high encounter rate" code, such as 94000130 FFEB0000 or 62110DC0 00000000, while holding the select and right buttons on the D-pad. You now have greatly increased the probability of finding certain Pokemon.
Enter a "rebattle Pokemon" code, such as 94000130 FFF30000, but alter the final three digits by entering Dark Lugia's Pokedex identification number of 0F9. Your final code should be 94000130 FFF300F9. This will ensure Dark Lugia appears in your next encounter instead of a random Pokemon.
Walk around in a patch of tall grass until Dark Lugia appears. With the increased encounter rate it shouldn't take more than a few steps.
Use Pokeballs to attempt to catch Dark Lugia just like any other Pokemon if you're feeling ambitious. Otherwise enter a "100% catch rate" code, such as 92247612 00002801, to ensure that even a standard Pokeball will catch Dark Lugia on the first try.
Step 4 can be repeated with any Pokemon by adding that specific Pokemon's Pokedex identification code instead of Dark Lugia. ID codes for other Pokemon can be found online.
If you try to catch Dark Lugia without using the catch rate code, you will need to reduce it's health considerably or any Pokeball you use will likely fail. Dark Lugia is extremely powerful. Don't attempt to fight it unless you have very high level Pokemon in your party.