The cave goblin skull is an item on the Odd Old Man's wish list during Rag and Bone Man II. It is dropped by cave goblins. After the quest is completed you can no longer obtain this as a drop. The bone must be put into a pot of vinegar and boiled to create a polished cave goblin skull before giving it to the Odd Old Man.
Monster Combat level Quantity Rarity Cave goblin guard 24; 26 1 Uncommon Cave goblin miner 11 1 Uncommon