They are notorious Conjurers – in fact stories about them giv us much of our knowledge on *how* conjuration is done.
Their relations with certain Daedra – he majority of things conjured and the easiest to conjure are from oblivion – also give them some insight into the divine diseases of Lycanthropy and Vampirism
She use to be a witch, she said she gave up witchcraft a long time ago. Janus Hassildor references her to being one of them. Hinting that one still exists in Cyrodiil's borders.
We're in the right region for the Glenmoril Wytches...
Them being tied more closely to Daedra than other mages implies that they draw their spells from the Waters of Oblivion (if we are to use lore that hasn't really been touched on since Battlespire).