How do you get a dusknoir in Pokemon Platinum?

This is very simple.

Firstly, if you want a very friendly and loving friend dusknoir, to get the friendship badge, capture a duskull.

Evolve it at level 55. Okay, now make sure you get a reaper cloth. Trade with someone while the dusclops holds the reaper cloth.

Ta Daa, le dusknoir!