The Violet Citadel is the central fortress of Dalaran which housed/houses the Eye of Dalaran, Chamber of Air, and a great spell library of the Kirin Tor — the ruling mage class of Dalaran. It is the capital city of the nation of Dalaran and was located on Cross Island.
The Violet Citadel is the location of the Grand Library. The Violet Citadel also contains the Arcane Vault which houses Dalaran's most powerful artifacts. It was from there that Teron Gorefiend stole the Eye of Dalaran and killed Sathera.
In the rebuilt Dalaran seen in World of Warcraft: Wrath of the Lich King, the Violet Citadel is home to Rhonin and his council of advisors — Vereesa Windrunner, Archmage Aethas Sunreaver, and Archmage Modera — who rule Dalaran and lead the Kirin Tor in its war against the blue dragonflight and Scourge. It is also the location of the Kirin Tor quartermaster who is located on an upper platform directly behind the four leaders.