This quest is automatically acquired upon completion of On the Loose.
Speak to Varric at The Hanged Man tavern in Lowtown. Varric reveals that he sold Bartrand's mansion in Hightown sight unseen, but now the new owners believe the property is haunted. Varric suspects that the lyrium idol is still there, or, if Family Matter was refused or never accepted, Varric suspects a non-specific relic is to blame. He asks Hawke to accompany him and investigate the mansion, believing that because Hawke is a mage or comes from a family of mages, Hawke is adept in dealing with "weird."
A new companion quest Haunted is unlocked even if Hawke decides not to help Varric.