Speak with Carendin Halgar in the Undercity.
Ah, [player]! I just received word from Carendin Halgar that he would like to see you for further training.
You should be proud of yourself, [player]. It's not every day that Master Halgar personally sees fit to instruct a young warlock.
You'll be able to find him in the Magic Quarter of the Undercity, near the Temple of the Damned.