It's one of the quests that have impact on the ending of the game. You will decide the fate of mages in Novigrad by completing or ignoring this quest. Sadly, no matter the decision you make, both endings are bad.
- Complete the quest (mages escape) - the mages will survive but the hostility against non-humans will increase,
- Ignore the quest - mages will be captured and killed during the slaughter in Novigrad.
It starts after the main quest Get Junior when, after the conversation with King Radovid, a passerby will give you a letter from Triss. After reading it, go to sorceress' house. During the conversation you can decide whether Triss should give the necklace to the house owners - let it happen by standing on her side. After a short conversation you will leave the building and three Witch Hunters will attack you.
Save Anisse and Berthold
Triss will tell you about two people that haven't managed to get to the appointed meeting. You can decide whether you want to save them. If yes, follow the sorceress to the pointed house. You must act quickly inside, otherwise the Hunters can kill the pair you are saving. You should have the Aard sign prepared and use it the very moment you enter the first floor. While walking down you can talk to building owners though it won't make any difference. Now follow Triss to the inn. If you decided to help those two, the inn owner will die and you will have to take care of three more Hunters in the inn.
Cleaning the sewers
Your task is to clean the sewers. Make sure Triss is following you, without her you won't open the closed door. Place the Necrophage oil on your sword, Muckixer (level 14-15) will be your main opponents in the sewers. Walk down the stairs and go to the nearest turn left - on the right side you can destroy the wall and take the Disglair sword from the chest. Then destroy the wall to make a passage - you will reach the library in which you can collect some books. Destroy another wall and keep going through the sewers until you reach the exit.
Before leaving you will be attacked by the Katakan - a vampire on level 14. When you reach the port, the final battle with Witch Hunters will await you. It will be easy because you will have some help. After the battle an important moment will occur: It's the last chance to tell Triss you love her and start a relationship (it will impact the game ending). You will have limited time to make the decision, hesitation (the middle option) will be considered as refusal.
Finally you will receive an proposition from Dijkstra. It will also start the optional quest A Deadly Plot in which you kill king Radovid - it will have impact on the game ending as well.