The Mage order hall, Hall of the Guardians, is a sanctum located within Dalaran, which, in Legion, has been moved to the Broken Isles. The Hall of the Guardians will be an integral part to pursuing the Mage Artifact Weapon and Mages will be able to return to their hall from anywhere, much in the same way you can return to your Garrison now.
Here is a preview, featuring some of the highlights of this upcoming feature. Pwncast’s Loremaster Lycan shares some history on the Mage class and the original Mages on Azeroth- the Kaldori. Enjoy!