Boethiah's Calling

Walkthrough
This quest begins either by reading the book Boethiah's Proving, which can be found in various locations around Skyrim.

Find the shrine of Boethiah
Or by going directly to the Sacellum of Boethiah, which is a shrine located east of Windhelm, in the mountains past Traitor's Post.

Make your way to the Sacellum of Boethiah.

Find the cult of Boethiah
Once there you will stumble into the members of the cult.

Lead someone to become trapped by the shrine and slay them
There you will be met by a Priestess of Boethiah. The conversation will begin automatically. She will tell you to bring someone to the top of Sacellum.

Find someone who you can hire out for 500, a thane's assistant, or if you are the leader of a major guild, ask someone you do not care about to follow you. Anyone is fine, the only obstacle is your conscience.

Once back at the Sacellum make the victim stand close to the giant pillar sitting before her altar. Tell them "I need you to do something" then activate the pillar. At that moment the person will be pulled toward it. Once pinned, kill that person.

Slay the other cultists
After that, Boethiah will begin to speak to you. Once the conversation is finished she will tell you to be the last one standing in a battle with the rest of her worshippers.

Win the battle and she will take control of one of the dead followers and tell you to kill her old champion who is at Knifepoint Ridge west of Falkreath.

Slay everyone at Knifepoint Ridge stealthily
If you are not very good at sneaking, you can simply run in and kill them all. You must then find the Champion of Boethiah who is located at the the very back of the mine.

Retrieve and equip the Ebony Mail
Kill him, take the Ebony Mail and then wear it.

Boethiah will then tell you that the Ebony Mail is yours and that you are her new champion. (If you did not already know being her Champion is just an honorary title there is nothing more you must do for her).

Boethiahs Ruf