You will get the info from the Mages Guild or the Telvanni, but only if you're a member of the respective faction of a suitable rank (mouth/conjurer +) and the NPC has at least 70 points of disposition. This part is skippable though, as you can go directly to Baladas and ask him about it - the only requirement is a disposition of 90+.

The topic is initially given by asking the abovementioned NPCs about "latest rumours" (though it is not 100% guaranteed to appear at the first time).