How to get to Mournhold - Elder Scrolls: Morrowind
To reach the city of Mournhold, where the entire Tribunal Expansion pack takes place, you'll first need to travel to the town of Ebonheart. You can find Ebonheart to the west of Vivec and you can reach this town by boat from Vivec or by using Divine Intervention when you're standing in Vivec. Divine Intervention will put you right outside the Grand Council Chambers which is where you'll find Asciene Rane (pictured above).
If you're coming by boat, in the western portion of Ebonheart you'll find a large building called the Grand Council Chambers. Speak with Asciene Rane inside of this building and tell her transport to Mournhold to be teleported to Mournhold. Once you are here if all you care about is stopping the Dark Brotherhood Attacks on yourself talk to any High Ordinator or Royal Guard about the Dark Brotherhood.
Note: If this is your first time visiting Mournhold you'll need to speak with Apelles Matius in Ebonheart about the Dark Brotherhood and Asciene Rane before you can be teleported to Mournhold. You can find Apelles Matius before the long path leading up into the Grand Council Chambers.