Bonehilt Dagger  

 

This drops from Dragoneater, who also commonly drops the Mask of Snake Leather, but the Bonehilt Dagger is much more rare.

Since the Mask of Snake Leather is lore, your best bet for farming this and for making some money in the process is to kill Dragoneater and loot the mask, then kite another Dragoneater till you're somewhat close to the merchant at the encampment to the south and kill it there. Sell the first mask (about 47 plat) then loot the second dragoneater and sell its mask. Since the second corpse is really close, this will save you a bit of running. Repeat.