When you enter the canyon city of Markarth for the first time, you witness a Forsworn attack on one of the stall holders in the city’s marketplace. Go out of the city and come back in again, and head down the right side of the thoroughfare, along the canal, until you meet two men standing outside a dwelling deep in conversation. Approach them and listen.
A cowled priest named Vigilant Tyranus is asking a Nord called Yngvar whether he’s seen any strange lights or unusual noises emanating from the house. Yngvar says he hasn’t, and at this point Tyranus turns and asks you similar questions. Reply however you want, and Vigilant explains that he’s what’s known as a Vigilant of Stendarr; one who locates areas believed to be used for Daedra worship and exorcises any Daedric presence within them.
Ask him if he need any help, and he asks you to enter with him into the abandoned dwelling he was just asking Yngvar and yourself about…
From the conditions inside the house, someone has obviously been here very recently. Tyranus is startled by an odd sound and opens the door, as a basket appears to role across the room in front of you both without anyone having touched it. You can then hear a faint moaning and a deep rumbling sound coming from somewhere nearby, and you perceive that a strange vapor is hanging in the air around you.
Tyranus is now certain that something is amiss here, and he descends a nearby staircase taking him deeper into the house. He shouts for whatever entity is present to show itself as candles illuminate the gloom around you both. You suddenly hear a voice inside your head that tells you to open another door nearby, but when you try to do so you find that it is sealed shut.
When objects start to fly around the room, Tyranus decides ne needs to withdraw from the area to go and find additional help. This is no ordinary Daedra. Follow him, as he makes his way to the entrance room, where he begins to slow down, seemingly affected by some unknown force. You then hear a growling voice that tells you to crush Tyranus.
You now have two choices: you can obey the command of the disembodied voice and attack Tyranus, or you can wait, hoping that he regains his composure. If you wait, he’ll eventually be completely overcome by the strange presence and attack you. So either way you have to kill Tyranus, but if you wait until he’s compelled to attack you, you’ll be acting in self defense and won’t be doing anything that goes against the behavior of a noble hero, if you happen to be playing your character that way.
When Tyranus is dead, the voice congratulates you, and instructs you to descend farther into the house to claim your “reward”. Go back down the stairs to the sealed door. It is now unsealed. The candles appear to burn brighter as you turn the handle to go inside. The voice beckons you into the bowels of the building as the chamber begins to tremble. Search behind the shelving to find a hole in the east wall. This leads into an earthen tunnel to a long-forgotten altar, upon which lies an ornate, rusty battle mace. Pick it up.
Interacting with the mace triggers a mechanism that causes spikes to spring up and imprison you. The voice then reveals itself to be Molag Bal, the Daedric prince of corruption, and the rusty mace is his Daedric artifact, the Mace of Molag Bal. This alter of worship was desecrated by a priest of his rival Boethiah, the Daedric Lord of plots, and the mace was left to decay - until now. Molag Bal wants the priest responsible to willingly turn to his service and surrender his soul. You have no choice but to agree to help as it’s the only way to free yourself. You don’t have to go through with it once you’re out of here if you don’t want to.
Working as the agent of an evil Daedric prince might be something you do not want to do because it contradicts the morality you’ve adopted for your character if you are playing the game as a ‘good guy’. In which case you don’t have to go any further with this quest; you can just ignore Molag Bal’s demand. But if you want to keep and use the weapon back on that altar permanently, which is probably the best mace in the game, you have no choice but to complete the Daerda Lord’s task.
If you want to complete this quest and be able to use the weapon, but are committed to your role-play of a good protagonist, you can perhaps justify doing so with this line of reasoning: The agent of Boethiah is himself a Daedra worshiper who would very likely, and possibly already has done evil things on his masters’ bidding. If he was forced to surrender his soul to Morag Bal he’d only be switching one Daedra for another, so you wouldn’t be delivering an innocent bystander into the clutches of the Daedra Lord of Corruption. You did agree to help Molag Bal, who then spared your life and granted you your freedom when you were his helpless prisoner, and if you don’t honor that bargain, he might some of his own mortal servants to come and try to kill you.
If you decide to honor the agreement you made with Morag Bal, then look at your world map and make your way to, and then through, the dungeon that the priest is lurking in until you find him. His name is Logrolf the Willful, and he’ll usually be found tied up and helpless. Tell him that you’re here to rescue him.
He’s immediately suspicious, as no one knew where he was when he was taken. He asks who sent you. You can (Pursuade) reply that his master Boethiah sent you, or (Bribe) ask him why it matters, as there’s gold in it for him if he comes, or (Intimidate) you can just tell him the truth to scare him: you were sent by Molag Bal.
When one of those options is successful, untie his binds. Logrolf has some other business to attend to, but it’s okay to let him go off on his own, as eventually the subtle will of Molag Bal will pray on his mind, causing him to return to the house in Markarth.
Journey back there yourself and enter the house and you’ll find Logrolf already in there. He runs down into the depths of the building and makes his way to the altar. As he approaches it, Molag Bal springs his spike trap and Logrolf is trapped. The pries is not perturbed, however, because he has been here, defied Morag Bal and escaped with his life before. Molag Bal tells him that he has a “champion” present this time, and grants you the use of his mace. Pick it up.
Stand between the spikes, and bring the mace down repeatedly on Logrolf. He yells that he’ll never submit. Eventually, you bludgeon him to death. Molag Bal then resurrects him from the dead and tells you to strike him again. After a coupe more vicious swings, Logrolf’s will collapses and he gives in to Molag Bal, promising to pledge his soul to the lord of corruption and forsake the weak and pitiful Boethiah. Molag Bal is satisfied… once you kill Logrolf again to deliver his soul to its new owner.
After all this is done, Molag Bal will present you with the true power of his mace as a reward before departing this plane of existence and returning to Oblivion.