Actually, this is the same place as in SoulSilver- the Whirl Islands. You'll need to get the Silver Wing from a man in Pewter City, and then you just run in and begin the quest for the Sea Guardian Pokemon!
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You need Ho-oh and Lugia
Lugia learns Sky Attack at level 99.
it wil never come back...
If you want a Lugia egg, first get the actual one. Then get a Ditto and put the actual Lugia and the Ditto in the day care (located south of Goldenrod City). Wait for them to have an egg and you'll have a Lugia egg.
A person in Pewter City will give you the Silver Feather, then go to Whirl Islands. When you get to the end of the cave you will see Lugia.