This is the shrine quest where you have to start by sacrificing a follower to the shrine. The first bug that occurs is that apparently dark brotherhood followers can not take part in this, when you tell them to use the shrine they just don't, they stand there looking at you.

I thought this might just be a problem with the dark brotherhood followers so I decided to find someone else, I hired the mercenary in Whiterun and brought her up to the shrine instead, and we get a little further. She will use the shrine and get trapped by it, when you kill her Boethiah comes and uses her body. The next quest objective is to speak to the Boethiah through the body, but every single time you talk to him he says the same initial line, complaining about how he's in a human body. He endlessly repeats this line and there's no way to progress further in the quest line.

Just thought you should know.