The crystal trinket is a reward from Arianwyn for completing Mourning's Ends Part II. It can be used to enter the Temple of Light after the quest. This requires mourner gear also, since the entrance is via the Mourner headquarters' basement in West Ardougne.
If you try to enter the temple after completing the quest without the trinket in your inventory, you will be bounced back and get the message "A strange force blocks your path." When crafting death runes through the Abyss, however, this is not necessary.