From Diablo Wiki
The mage clans of Kehjistan are famous for their knowledge and power. They are diverse and numerous.
On the eastern edge of the Twin Seas lie the emerald jungles of Kehjistan. Several thousand years ago, hunter-gatherers found themselves drawn to the lush, fertile rain forests and bountiful game inhabiting Kehjistan.
But there was something else; Kehjistan's geography was rife with nodes of magical energies, and eventually those with innate, arcane abilities began to sense and recognize these forces, gathering in groups to settle these special areas. It was here that the original mage clans, the Vizjerei, the Ennead and the Ammuit, were founded.
Unfortunately the history of Kehjistan is not a peaceful one, and the Mage Clan Wars that erupted nearly destroyed the clans. This war between the different clans ranged for a long time in the mage clan wars, but they have now put most of their differences aside.
The biggest and most powerful mage clan is the Brotherhood of the Vizjerei.