Arcane Vault

The Arcane Vault is the location that the Kirin Tor keeps all of it's most prized artifacts and treasures. The vault is located somwhere within the Violet Citadel. Its exact location in the citadel is a closely guarded secret among the Kirin Tor. It is considered to be one of the most magically secure locations on Azeroth.

Overview
The Arcane Vault was constructed as a place to safely secure the powerful magical artifacts the Kirin Tor was collecting. It has some of the most powerful wards and defensive spells portecting the vault from those who would seek the treasure within. It has only been broken into once by the notorious Teron Gorefiend he managed to get in and steal the Eye of Dalaran. It has since been returned thanks to the efforts of the Violet Eye and the Magus Senate of Dalaran.

Reference
Arcane Vault  article on Wowpedia.