From the enchantress
Player: Do you recall more now?
Enchantress Eirena: Yes, faintly. My sisters and I were once mere servants in the house of a great Vizjerei lord.
Notice here how she say's she was a servant in the house of a Vizjerei Lord? Remember that.
Vizjerei were one of the 3 major mage clans, they specialized in demon and demon accessories.
Okay, now, something else to remember now....
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.
It wasn't till the sin war that it became knowledge that these were demons they were summoning. Remember how the enchantress referred to them as demons?
Malthael is _not_ the prophet, yet it is still Inarius who we call the prophet.
If you wish to know where malthael is?
I'll give you a hint. Trang-Oul