Skyrim Sidelines- The House of Horrors: Sic Semper Tyranus

Moments after you try the door that leads further into the house (out of hope if you already tried the front door), the room darkens substantially, an echoing noise fills the house, and the various objects begin flying around.  Be careful, as they can hurt you significantly.  Startled and frightened, Tyranus turns to flee so he can go get help, as this is clearly not just any Daedra.  As he nears the entrance, Tyranus slows down, allowing you to catch up.  As you do, a growling voice will tell you to 'crush him'.

If you like, you can wait, but Tyranus won't manage to get out and will quickly deteriorate to the point of actively attacking you until one of you dies.  Alternately, you can simply take the initiative and kill the hooded Vigilant.  Once he is slain, you can resume passing through doorways in the house.  At the deepest part of the building you will find that the locked door is now unlatched and the light returns to the area as you open it.

Follow the gravelly voice into the deepest part of the stonecarven home and you will find a tunnel into the mountain hidden behind a bookshelf.  At the back end of this tunnel is an ancient altar, holding an old and rusty mace.  You will need to pick up the mace, at which point huge ebon spikes lunge out of the ground, surrounding you.  The voice returns again, amused that you think you could get such an easy reward.

You are now locked in conversation with Molag Bal, the Daedric Prince of Corruption.  As you meander through the condescending conversation, the mace and altar will be mentioned.  Molag recalls when the mace and altar ran with the blood of the weak, but Molag's rival Boethiah sent a priest here to, essentially, desecrate the altar.  In order to restore both location and weapon to their former state, the Daedric Prince wishes the priest to willingly give Molag Bal his worship and soul.  As there's no other way to escape the ebon cage, you will have to agree to do just that.

This prompts a new marker on your world map- travel there to find the priest, Logrof the Willful, stuck in a dungeon (usually captured and bound).  Make sure you can retrieve him safely (usually by killing whatever bandits have captured the guy) and then talk to him.  You will need to find a way to convince him to follow you back to Markarth- you can feign being a follower of Boethiah, offer him money as a reward, or if you're intimidating enough you can use the truth to frighten him into following you.