They were first introduced in the original Diablo game in the Sorcerer character class, and more of their history was related in Diablo II. Hired Mages in Diablo III also come from this clan. Eirena and her sisters were originally slaves to the Vizjerei mages.
Jerhyn's palace in Lut Gholein is an old Vizjerei fortress, which is why the portal to the Arcane Sanctuary is located in the palace cellar. The character Drognan in Lut Gholein is actually a former Vizjerei mage, but there are no details as to why he is no longer one.
The Vizjerei Clan was in fact one of the three primary (and most powerful) Mage Clans on Sanctuary. Due to the actions of a young and promising mage - an acolyte claiming to possess the power to command the spirits of the dead - the Vizjerei Clan was also known as the Spirit Clan. It was later revealed that the 'spirits' were in fact demons, making the Vizjerei one of the first Mage Clans wielding demonic magic. They used the macabre armor to shackle and control the summoned demons.
While the Vizjerei sorcerers had a long history of practicing the elemental magics, they are most notable for the two renegade magi, Horazon and Bartuc, who experimented with demonic magic that tore the Vizjerei apart. Bartuc sought to ally himself with the forces of Evil while Horazon enslaved demons to his will. In the end Horazon realized they were both being duped by Evil and gathered his forces to kill Bartuc. The Vizjerei worked with the Priests of Rathma to ensure that Bartuc would not be able to rise again. Horazon then went into self-imposed exile by becoming the Arcane Sanctuary, while Bartuc's head and body were separated and sealed into two different caves. (He later appears in the form of a Council Member as a minion of Baal at the end of Act V.)
Following this was the establishment of the Viz-Jaqtaar—also known as the Order of the Mage Slayers, or Assassins—who were to act against any magi who might achieve a similar depth of corruption.
After Diablo was defeated below Tristram and Andariel was slain in Khanduras, Horazon was still thought to have been alive, though it was found to be a mere pretender—a Vizjerei sorcerer who had returned from Tristram and had been corrupted by the Arcane Sanctuary.