You need an event.
Darkrai
No.
Nope
you have to get th mysrery gift to capture darkrai and go to the hotel inn a man will greet you there and you will fall asleep ,and poof you will apear in new moon island and have a chance to capture darkrai.
action replay
Darkrai.
Ok the answer is yes, but only throught the darkrai event
Darkrai is a Pokemon from Generation IV, and Pokemon ruby is a game from Generation III , so you can only get darkrai in Pokemon Platinum, Diamond or Pearl
You dont catch Darkrai in Pokemon Pearl. You can catch it in Pokemon Platinum at level 40.
In harbor inn
with a special event
No you don't have to.