To finish the quest, simply visit any Shrine of Stendarr.
Curse of Molag Bal is a 50% weakness to fire, and it prevents you from maintaining divine blessings **. The mace will immediately disappear, Logrolf will be released (he runs away and you'll never see him again), and you'll be cursed. If you don't want to murder an old man, just throw it on the ground right away *. If you want to have the mace, complete the quest as normal by killing Logrolf (repeatedly). Now when Molag Bal gives you the Rusty Mace to beat Logrolf with, the front door will lock at this point, forcing you to make a decision.
That's just horrible, so I've made a new option. There is no option to refuse and free the guy - the nicest thing you can do is leave him there to rot. A good player would still want to save him, but if you do that and follow him, Molag Bal forces you to beat him to death. Then you're told another guy is imprisoned. You get suckered into the quest as a mission of good, then forced to kill a guy. The The House of Horrors quest is terrible for good players.
