Head inside the abandoned house with Tyranus and he’ll note that someone is indeed living here. However, when he heads further in things start to get FREAKY. Items will come to life and throw themselves at you. At the back of the house is a door you just can’t open. Try to open it and Tyranus will run back to the entrance for help. Coward. A voice will tell you to kill him. Now, you can either listen to the voices (you crazy bastard you) or try to leave the house yourself. You’ll be unable to leave and Tyranus will attack you (so you may as well kill him first). Loot him afterwards and the voice will claim to have a reward for you further into the house.
Of course it would be all TOO easy to just get a reward, no? Head deep into the house and tunnels (as the door is unlocked now) and you’ll see an altar at the very end. Interact with it to be trapped! The voice will introduce itself as Molag Bal. Answer his questions however you wish, and in the end you will be tasked with finding the priest of Boethiah (his rival) and making sure he gets to this house safely.
Exit back to Markarth now and check your map. You’ll see that the priest is located at a place called “Hag’s End” (Which I had already CLEARED at this point in the game, which leads me to believe the PLACE where the priest is being held at is RANDOM)…
So head to wherever the objective points to for you and travel there. Kill any enemies you may find and talk to Logrolf. Tell him you are here to rescue him. From here, you’ll either have to PERSUADE, BRIBE, or INTIMIDATE him into returning, so try all three options until you succeed.
Once he is free he will head to the abandoned house, so return there yourself now. Head down to the altar where you will see Logrolf trapped! He claims that they’ve been down this road before and he’s won, but now Molag has YOU. You will automatically be equipped with the rusty mace as Molag has you beat poor Logrolf into submission. Beat him until he dies, and Molag Bal will bring him back from the dead. Beat him to death again and Logrolf will finally submit to Molag Bal and pledge his soul to him. And in return, Molag will tell you to KILL him. Haha, go ahead and kill Logrolf one more time. Molag will be pleased and will give you the MACE OF MOLAG BAL as a reward. Answer however you will and this quest will be over.
The mace is quite a prize if you’re into maces… it damages health, stamina, AND magicka! It can also fill soul gems. Sheesh!