Answer: You can't get it in Diamond.
You can't get it in Diamond or Pearl. You need to catch it in Emerald, Leaf Green, Fire Red, or other GBA games and then migrate Lugia to Diamond/Pearl. Note: You can't migrate a Pokemon from a GB Pokemon game.
you have to trade from pearl
it tells you how
in a Nintendo event
solacoen ruins
you have to get it at an event or get an action replay
BY HAPPYNESS IN DAY IF IT is umbroen is in night
No, you cannot get Lugia on Pokemon Yellow.
above jubilife city in the grass by using Pokemon Radar
im pretty sure it evolves certain types of Pokemon, you will have to find the Pokemon
Shadow Lugia is a fake pokemon. You can not obtain a Shadow Lugia in any Pokemon Game (so far). Shadow Lugia is just another rumor pokemon.
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.
lugia isn't in Pokemon leaf green