How to get this quest
Talk to the demonologist in the underground city.
Tips to find the answers
The answer to question 1 is already given during the story. Questions 2 to 4 are answered by NPCs inside the underground city (one of them is in the west part of the city) and its bar. Failing to answer one question doesn't fail the quest. Talk again to the demonologist allows to get another try.
The answers
The special answer to question 3
It is: Dolus.