The House of Horrors

Background
''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.''

Walkthrough
This quest can be obtained shortly after entering the city of Markarth. Tyranus, a Vigilant of Stendarr, will ask the Dragonborn to help him search an abandoned house for evidence of daedric activity. Upon entering the basement, Tyranus will proclaim that "This is no ordinary Daedra!" and run for the door, only to find it locked. Molag Bal will speak and command the Dragonborn to kill Tyranus. After he gives the command to kill Tyranus, the room starts to shake, the lights become dim, and objects move as if by themselves. If the player chooses not to kill Tyranus immediately, Molag Bal will convince him to attack. A Scroll of Calm will not prevent Tyranus from attacking, but casting Harmony will temporarily cease his attacks until it runs out, at which point he will resume his attack. Voice of the Emperor used by Imperials will calm Tyranus. Either way, Tyranus must be killed in order to progress any further. Afterward, journey down to the altar of Molag Bal in the basement.

Molag Bal entraps the Dragonborn in a spiked cage while activating the mace and says that he wants revenge on his rival Boethiah (another daedric prince) and her followers. He wants Logrolf the Willful, a priest of Boethiah who has been desecrating Molag Bal's altar in Boethiah's name, lured to him so he may break his spirit.

The location of Logrolf the Willful is randomized; any Forsworn camp may host him. Travel to his location and free him. If the Quest No One Escapes Cidhna Mine has been completed, then the Forsworn holding him may not be hostile. If all enemies at his location are dealt prior to releasing him, he will immediately return to the abandoned house in Markarth.

Once back inside the abandoned house, follow Logrolf back to the altar. He will become trapped in the same cage that the Dragonborn stumbled upon earlier. Molag Bal will then grant the player his Rusted Mace and instruct them to break Logrolf's will. Attack Logrolf (any means of attacking can be used; the Mace doesn't have to be used) until he submits to Molag Bal. Once he submits, the Dragonborn is instructed to kill him. Do so to receive the Mace of Molag Bal and finish the quest.

Trivia

 * This is a good opportunity to level up Restoration and attack skill(s) without fear of reprisal. Alternate between hitting Logrolf until his health is close to zero, and healing him back to full health. Keep doing this for as long as desired, but be sure not to kill him and that he does not submit too soon.
 * One can also level up the Sneak skill at the same time by sneaking at the same time.
 * The whole intro, with Molag Bal speaking, can be skipped if the Vigilant is killed immediately upon entering the house.
 * If Tyranus can't be found, he is known to spawn near the stables outside the city of Markarth and his dialogue remains as if he's near the abandoned house or ask a innkeeper if he has heard any rumors lately and this will automatically give the location of Tyranus when the rumor of the abandoned house is mentioned.
 * There is a Poltergeist reference when Tyranus decides to flee the house and get help. The table near the exit of the house has chairs stacked on it similar to a scene in the movie.
 * It is possible to find Fjotra and Logrolf the Willful in the same pen when performing the quest The Heart of Dibella if doing these quests simultaneously.
 * It is possible to obtain multiple Hooded Black Mage Robes from Logrolf during this quest, by quickly searching Logrolf after you kill him. When he is revived by Molag Bal, he will respawn wearing a new set of robes.
 * To fail this quest, refuse Molag Bal, and kill Logrolf in the pen
 * If Dawnguard is installed,then two master vampires will spawn inside the house.
 * One of the possble locations that Logrolf may be held captive is Druadach Redoubt, which is where Madanach and his companions set up their base of operations if the Dragonborn chose to aid Madanach during the quest No One Escapes Cidhna Mine. As in this event the Forsworn are non-hostile to the Dragonborn, it is possible to simply walk up to Logrolf and untie him without being attacked.
 * In the PC version of the game, when Molag Bal demands you kill Tyranus and the house begins to shake and hear loud noises. You can open the front door using the open command via the console. Tyranus won't leave, even though he knows his life is in grave danger. But if you go outside the shaking and sound effects will continue to happen regardless where you are and how long it has been. Nobody comments on it.