An unholy mould is used to make silver bars into unholy symbols using the Crafting skill. This may be done at any furnace across RuneScape. Players must complete the Observatory Quest before they can obtain an unholy mould, thus making it untradeable (unlike the holy mould). A mould can be acquired for free by talking to the Spirit of Scorpius (you do not need the ghostspeak amulet to talk to him) at the Grave of Scorpius near the entrance to the Ourania Cave, which is north of the Observatory and west of the Khazard Battlefield.
The spirit will only give a mould to a player if the player does not have one. However, multiple moulds can be obtained from the spirit by using the drop trick.