The altar
The Altar of Molag Bal is a shrine to Molag Bal located in the basement of the Abandoned House in Markarth.
Related quests
In the quest "The House of Horrors," Molag Bal asks the Dragonborn to find Logrolf the Willful to cleanse his altar, as it was desecrated when Boethiath's priest offered on it. Once the task is done Molag Bal rewards the Dragonborn with the Rusty Mace, which has been cleansed into the Mace of Molag Bal.