This quest is not available in game.
Speak to Aelthalyste in the Undercity.
Ah, a wonderful day for you, <name>. I can see by the lessons you've learned that the time has come to send you back to Aelthalyste. There are things only she can teach you, and even then, only if you prove worthy.
I will not spoil the surprise for you, but I remember when I was a young priest learning about true power... divine power. Yes, those were the days.
Go back to Aelthalyste in the Undercity, <name>. She will be awaiting you in the War Quarter.