From the RuneScape Wiki, the wiki for all things RuneScape
A Death tiara is made by combining a Death talisman with a tiara. In order to combine it with a talisman, simply use the tiara on the Death altar with a Death talisman in your inventory. Doing so earns you 50 Runecrafting experience.
A death tiara can be worn in the head slot, allowing you to left click on the altar to get inside, saving you an inventory space.
These can also be obtained as loot from Dragon implings which require level 83 hunter to catch.