Striker on 5/4/2005 at 04:51
Mournhold is part of the Tribunal Expansion. Speaking to the teleport guy in Ebonheart is enough to stop the Assassins from attacking when you sleep. (Teleporting there doubly so) This means that you won't be attacked again.
It's probably also worth noting that the assassins will attack you to a max of 8(ish) times. The number of times is dependent on your level. Once this number has been reached, they won't attack you again. With the XBox version, they don't start attacking until level 5 or so. I don't think this is the case with the PC version.
Tribunal is much harder than Morrowind, so it's probably best that you don't attempt it until you are either level 20ish, or you have some awesome artifacts. :)