The dark brotherhood is mentioned quite frequently in this quest, now It may be because I have DB, but you will need those skills on some parts of the quest in my opinion, and Ras will accused one of the actions as a DB thing.
P.S Dark Brotherhood is needed because a previous quest that you do lines up with Naryu thinking the DB sent a killer after her and in the end she needs to blame her kill on the DB so you come in handy to help her out. You show yourself after she kills the person in Kvatch Castle so they believe the DB did the murder and not the Morag Tong.