After recieving this side-quest from Samara, head to Omega and go in to the Afterlife club to talk with Aria. She will inform you of where the most recent murder took place.
Head over to the apartment (a previously locked door can now be opened, over by where you went to recruit Mordin). Talk to the dead girls mother. Go in to her room, examine all the journal entries and the other 2 items in the room...
Head to the Afterlife VIP party area, tell the guard that Jaruut suggested you come here. Enter the party. Talk to the guy immediately to your left about the band hes talking about, leave him.
Fight with the turians that are planning to steal, talk to the man asking you to help with his friend by using the 2 codewords in a coversation with her, punch the guy trying to prostitute the dancer.
After all this is done, you should be approached by Morinth. Once in a dialogue with her talk about art (mention the artist that was in the girls journal), talk about travel (mention the ideas of danger and violence is a means to power) and talk about music (mention the band that the guy at the club enterance bugged you about). This should land you back at her apartment.
Before talking to her, grab the upgrade on the wall and feel free to look at the other various items around the room. Talk to Morinth. I then proceeded to land all the renegade convo options (think you can do the paragon option for the same effect) to stay on her good side. Eventually she will try to hypnotise you, reply with the renegade/paragon option and Samara will enter the room. They will have a duel and you will be left with the choice of killing Morinth or Samara.
To be on the safe side, I chose to kill Morinth as I didnt know if chosing Samara will net you the achievement...But thanks to SalaciousPorks, I now know that chosing either will net you the achievement...Once one of them is dead and everyone finishes talking, the achievement should pop. Nice job!