Rohan the Assassin at Light's Hope Chapel in the Eastern Plaguelands will make a Bonescythe Helmet if you bring him the following: 1 Desecrated Helmet, 15 Wartorn Leather Scraps, 8 Cured Rugged Hides, 1 Nexus Crystal and 75 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.