An unholy symbol is a silver symbol blessed to imbue it with the unholy power of Zamorak. Players can make unholy symbols by using the Crafting skill, with a Crafting level of 17. First, a player must use a silver bar with a furnace while they have an unholy mould in their inventory and must have one free inventory space. This will give the player an unstrung emblem and they will receive 50 Crafting experience. Next, the player must string the symbol with a ball of wool, giving the player 4 Crafting experience and making the item into an unpowered symbol.
Finally, the symbol must be blessed by the Spirit of Scorpius at the Scorpius Shrine, north of the Observatory, or by using an Unholy book or Book of balance with it. To bless the symbol using one of the books, a player must have a Prayer level of 50 or higher, and blessing the symbol makes the player expend amount of Prayer points. Once blessed, the unpowered symbol becomes an unholy symbol and gains stat bonuses.
This item can be obtained when the player is chasing Solus during the Wanted! quest.
For an exhaustive list of all known sources for this item, see here