The wrath talisman is an item that allows players to enter the wrath altar where wrath runes are crafted. The wrath altar is located south of the Myths' Guild, which can be accessed after completing Dragon Slayer II. Making wrath runes requires level 95 Runecraft.
The talisman can be combined with a tiara to make a wrath tiara for 52.5 Runecraft experience at the wrath altar, by using the tiara on the altar. This makes the talisman disappear, but the wrath tiara can be used in just the same way and can be worn on the head, saving an inventory space when crafting wrath runes.
For an exhaustive list of all known sources for this item, see here