How can you join the dark brotherhood in Morrowind?

First of all you need to become a Vampire. Since you're asking this question, I'm going to assume you already did this. So, what you need to do is quite simple to join a clan. You need to go to a place where there are Vampires (Caves, Ancestral Tombs, etc.). Then you need to talk to all the Vampires until you find one that can give you quests. You can usually pick these characters out of the bundles of Vampires that are in some places because they always look really important.

The Categories of this question is so wrong.