Speak to Aelthalyste in the Undercity.
Aelthalyste show our people great respect by sending them back to me for more training, and now I do the same for her tribe.
You return to the Undercity, <name>. There, in the War Quarter, she be ready to teach you more 'bout your people and their ways. Go there, and do not wait long.
I would teach you Devouring Plague if you feel you are prepared, <name>. A spell of no small power, it will aid you in your coming battles. Practice with it--there will be tests later.
The entirety of this quest happens at Undercity