This quest is automatically acquired upon completion of the On the Loose.
Speak to Varric at The Hanged Man tavern in Lowtown. Varric will reveal that he has been trying to sell Bartrand's mansion in Hightown, but he failed because people believe that it's haunted. Varric suspects that the lyrium idol is still there and he asks Hawke to accompany him and destroy it.
A new companion quest Haunted will be unlocked even if Hawke decides not to help Varric.
After finishing this conversation, Varric may say "The truth is out there", referencing "The X-Files" famous slogan. The Talkative Man, another patron of the Hanged Man may say "The truth is out there," if clicked on.