Rohan the Assassin at Light's Hope Chapel in the Eastern Plaguelands will make Bonescythe Legplates if you bring him the following: 1 Desecrated Legplates, 20 Wartorn Leather Scraps, 1 Arcanite Bar, 8 Cured Rugged Hides and 100 gold pieces.
If you want Bonescythe made, you're going to have to supply the materials.
Enter Naxxramas, the big floating city of death in the sky up there, and find desecrated armor and wartorn armor scraps. Bring those back along with the materials to put everything together and you'll have your Bonescythe. Oh, and you'll need to pay me for my troubles... I'll supply the bones.
Be careful walking around in those things. You might cut yourself.
The entirety of this quest happens at Eastern Plaguelands