I only know of one I think theres more but you use it on dusclops to get dusknoir trade dusclops with reaper cloth to evolve it.
It helps you get a Dusknoir. They rock, too! I have that's lvl. 81!!!
The Reaper Cloth is to the right of Acuity Lakefront. When you use rock climb going to Acuity Lakefront go to the right and you will see a pokeball and it contains the Reaper Cloth.
Entrance of lake acuity and go right
Hello boy/girlz it ligth Achet ...
Trade dusclops, trade back, ta-da!!! Dusknoir!
If you use rock climb before you go in acuity lake go to the right then you find reaper cloth
If you have beaten the elite four, you can take the boat to the resort area, go to Route 229, and there is a tree that can be cut down hidden behind a tree, cut that down and go right a little, and there is the pokeball with a reaper cloth.
Just before you enter acuity lakefront, after you scaled the small ledge, go to your right, there will be an item there, thats the reaper cloth
You go to snow point city when you are at acuity lake you use rock climb go right you see a pokeball and inside its a reaper cloth
In font of acuity lake where you use rockclimb then go to the right and it will be there,the reaper cloth.