A cosmic tiara is a head slot item used in Runecrafting. It can be created by using a tiara on the Cosmic altar with a Cosmic talisman in the inventory. Doing so grants 40 Runecrafting experience.
When worn, the cosmic tiara allows access to the cosmic altar through a left-click option on the mysterious ruins that lead to it. This saves one inventory space that would otherwise be occupied by the cosmic talisman.