The death talisman is an item that allows players to enter the death altar where death runes are crafted. The death altar is located in the Temple of Light, which is discovered during Mourning's End Part II. Making death runes requires level 65 Runecraft.
Death talismans can be obtained by killing dark beasts with level 90 Slayer. They can also be obtained during and after Mourning's End Part II by giving the dwarf Thorgel, located at the entrance to the death altar, all 50 items on his item list. Thorgel will give one death talisman for every item list completed. Since completing item lists for Thorgel takes a very long time, this way of obtaining death talismans is not recommended.
The talisman can be combined with a tiara to make a death tiara for 50 Runecraft experience at the death altar, by using the tiara on the altar. This makes the talisman disappear, but the death tiara can be used in just the same way and can be worn on the head, saving an inventory space when crafting death runes.
For an exhaustive list of all known sources for this item, see here