Return to Hawke's Estate and check the letters. Accept the request from Meredith and set out for the Gallows. She will brief you about your mission there. Your quest is to find three escaped mages who are loose in Kirkwall. Speak with Elsa to learn the last-known locations of each of the Mages before setting out.
Begin by going to Hightown and speaking with his parents in their mansion (follow the arrow indicated on the map). When the conversation is finished, proceed to Hightown (Night) and talk to Emile. You can either turn him or let him go. Either way, he will depart without a fight.
Begin by speaking with Walter in Darktown. He will make mention of Evelina and give a few hints about her past. They will then flee deeper into the sewers, so follow them.
Shades and Rages Demons attack when you venture into the sewers. Use your best area attacks to kill the Shades before turning on the Rage Demons.
Evelina transforms into an Abomination, but she's not your garden-variety horror. She's mostly there to suck up attacks while shifting from Abomination to Desire Demon in an explosion of light. Don't get too close though, or you will probably get hurt. Head to Lowtown to find the final Mage.
Huon is by far the nastiest of the three Mages that you will encounter. After speaking with his abandoned wife in the Alienage during the day, return at night to find him engaged in acts of devilry. Needldess to say, you will want to take him out as quickly as possible with your strongest character and attacks. Return to Meredith in the Gallows to finish the mission.