What I can tell you is that you get attacked by a dark assassin once you do something bad to someone e.g. when you raid one of those random tombs or caves and kill everyone inside. You will get a surprise the next time you go to rest.
You'll be able to reach Solstheim by taking a boat ride from Khuul I think it was.
As for Mournhold I'm sorry I can't quite remember. I haven't played for a long time now. Have fun man. Good game.
