From Diablo Wiki
The female mage clan of Zann Esu is one of the oldest of the ancient clans, although little is actually known about them. Centuries ago, the fourteen powerful covens of Esu witches convened for the first time in generations. What they discussed is not known, but the witches left behind their former lives and, as a group, disappeared into the Eastern Jungles.
The exact location of their community is a mystery. Until recently, their only contact with the outside world occurred during the recruitment missions. Once every seven years, the Zann Esu visited certain families across Sanctuary. These families had one thing in common -- they each had a seven-year old daugther. Always good-natured and polite, the Zann Esu visitors would meet the girls, ask a few questions, and then leave. A select few of the girls were visited a second time and offered apprenticeships. The families of those chosen enjoyed good fortune for many years after.
The Zann Esu, or Sorceresses as they are generally known, are on a quest for purity, the pursuit of the perfect magic. They feel the other disciplines of magic are haphazard, and have instead chosen to focus strictly upon elemental magic. They mold the base elements into whatever magical forms they need -- threatening all other magic disciplines with obsolescence. In order to achieve perfection in these elemental transmutations, they choose only those daughters of Sanctuary with the highest level of attunement to the magical elements.
The Sorceresses believe that it is through the search for perfection that they will attain ultimate purity and ascend to their destined role as the most powerful mages in Sanctuary. For centuries they have studied in secret, perfecting their art and bidding their time until the Emergence of Evil. Then, they will face their greatest challenge, either proving the purity of their magic or fading from existence.
The Zann Esu oracles have decreed that the time of the Emergence is at hand. The destruction of the Prime Evils is to be the great test of their clan. Recently, Sorceresses have appeared mysteriously throughout Sanctuary to do battle with the minions of evil wherever they are found.
Zann Esu Organisation
Not much is known about the organisation of the Zann Esu, but they have oracles to tell prophecies.
The mage clan also has ties with other clans such as the Vizjerei, and if they receive exceptionally troublesome students, they can sometimes hand them over to the much more disciplinary male-heavy mage clan.