Go on a balcony that is located in the place where the wedding is. There, you can meet Dorian Gall.
If Ifan ben-Mezd is in your party then the man will talk to him. He wants Ifan to murder someone. Ifan will refuse. However, you can accept that request. You must kill Micheil Ros.
Micheil can be found on a floor of the mansion.
Speak with him. You can choose one of two options. The first one is to kill Micheil and the second one is to accept his counteroffer.
The quest goes almost the same in both cases. If you accepted his counteroffer then Dorian won't be on the balcony. However, if you killed Micheil then Dorian will poison everyone. He escapes after a short conversation.
Regardless of which option you chose, you must kill Dorian. Go to his house, you can find it near the bridge to The Black House. You can break in or simply destroy the door. There, you can find a furnace. Find a button on it. Press the button to discover a hatch to a laboratory.
The whole laboratory is full of traps. Dorian and his failed experiments are located in the last room. A short conversation ends with a fight. If you killed Micheil then the quest ends with Dorian's death. However, if you accepted the counteroffer then go to Micheil and receive your reward.
Reward: 112,100 experience points + 96,650 experience points (optional) + gold (optional)