Ultima Underworld II: Labyrinth Of Worlds

To commemorate the defeat of the Guardian's forces one year ago, you (the Avatar™) and the leaders of Britannia are invited to Lord British's castle for an evening of celebration. Suddenly, the Guardian strikes again, encasing the building in a gigantic blackrock gem. He offers the imprisoned heroes a simple choice: proclaim him ruler of all Britannia, or starve while his minions conquer the land.
Searching underground for an escape route, you discover a smaller version of the gem that surrounds the castle, and find that its facets are portals to other worlds. These worlds are also under the Guardian's domination, but with your help, the inhabitants may be able to throw off the evil titan's yoke.
Time is running out, food and water are running low, and an agent of the Guardian is killing off the imprisoned heroes one by one. You hold Britannia's fate in your hands once again.