The two expansions integrate into the main campaign, and "hooks" are placed in it to get you to the new locations. It is, however, your decision as to when you want to go to either of the new areas. I didn't much care for the Bloodmoon one myself and only did that once. Tribunal was another matter. You can pick up some really very good stuff there. Mournhold (the city of Tribunal) is a tough place, though, and not for low-level characters. On the other hand, you definitely want to visit well before the end of Morrowind.
Nightowl2
|