if this is the swirl islands . . <--
. . go to this one
You can't get Dark Lugia in Pokemon Silver, Gold, or Crystal. You can get Dark Lugia in Pokemon XD though. For Pokemon Silver, Gold, and Crystal it is just the regular Lugia.
In either the dark cave or the cave leading to azelia town, I don't remember, but you have to go deep into one of them and get the rarest water Pokemon (except for lugia) its Lapras.
Yes by trading.
Dark Cave
the dark cave can be entered on route 45 or route 46
Dark cave.
You can find Waterfall in the Dark Cave.
in the dark cave near blackthorn city
You cannot. Dark Lugia is only available in Pokemon Colloseum.
you go to the third gym you go up you go to the grass and you randomly walk around and voila! dark lugia if you want shiny do the same thing but release your mew and mewtwo and there is shiny lugia.
dark Pokemon are
Dark/Shadow Lugia is only available in the Gamecube game Pokemon XD: Gale of Darkness.