The Darkrai event allows access to Fullmoon Island to catch him in a dream.
but if you missed that (like ME) you can catch a Stunky under the Cycling Road. And Train it up to be a Skuntank like the Galactic Commander Jupiter.
Fighting type and bug type are strong against dark type Pokémon.
Darkrai is a dark type of Pokemon. You can catch one in Pokemon Diamond version on Newmoon Island but you need a membership card.
Ghost type is weak to Ghost type and Dark type moves.
Weak: Psychic Strong: Fighting
Sigilyph is weak against Rock, Ghost, Electric, Ice, and Dark type Pokemon.
Fighting type and bug type are strong against dark type Pokémon.
You can't. There is no such thing in any legit version of Diamond or Pearl.
I would choose a Dark or Ghost type
=====================DARKRAI===========
Have a strong fire type pokemon.
Dark type are very strong to physic pokemon. Grass,fire and water are weak(not very strong).
only fighting type Pokemon
the moon stone.
Poochyena (sometimes wrongly spelled as Poocheanna) is a dark type of Pokemon. You can get one in Pokemon Diamond by breeding a Mightyena.
Darkrai is a dark type of Pokemon. You can catch one in Pokemon Diamond version on Newmoon Island but you need a membership card.
fighting types and bug types are the most effective types on dark.
Ghost type is weak to Ghost type and Dark type moves.