Ok I'm a really good assassin and I want to join the brotherhood ...recently they tried to kill me when I was getting a deadric artifact ... So what's happens now can I still join or what?

DB Assasins are just another random enemy. Does not mean the DB want to talk to you yet. In fact, the DB sent some Assassins after me once I became head of the guild.

You have to do a quest for a boy in Windhelm and that will start you on the path to the Dark Brotherhood.

What you experienced, was a random encounter, after someone, supposedly performed the Black Sacrament about your head.

Your actual invitation to the Dark Brotherhood comes after a certain quest -should you choose to join them, ofcourse.

To join them...