Coronet of the Lu'minellia

The Coronet of the Lu'minellia is an item located in the Hall of the Guardian. It is the headpiece of the Sunstrider dynasty's royal suit of armor, Lu'minellia, which was last seen on Anasterian Sunstrider's person during the Scourge invasion of Quel'Thalas.

Source
The coronet is found in the Cache of the Sin'dorei in the Hall of the Guardian, near Uthel'nay.

As a quest objective
This item is an objective of The Vault of the Tirisgarde