The Abandoned House in Markarth is a seemingly uninhabited and empty home which the Vigilants of Stendarr believe is being used for Daedra worship. A Vigilant will be asking passers-by for their assistance in investigating the house.
There are numerous chests and containers, an unowned bed, as well as services available close by at the entrance to Markarth. It appears that the chests and dressers inside the house do not re-spawn items, and can be used for storage. On the bottom floor, a cave can be found, leading to a shrine of Molag Bal.
On the outside of the house, there is a Shadowmark that indicates that the house is empty.
I have met Molag Bal, the Lord of Domination. He has asked me to cleanse his altar in the abandoned house in Markarth by luring a priest of Boethiah to him.