Ultima Underworld II: Labyrinth of Worlds

In an attempt to conquer the world, The Guardian encases Castle Britannia and its occupants--including the Avatar--in a giant blackrock gem.

The game takes place one year after the events of Ultima 7: The Black Gate. To celebrate the defeat of the Guardian, Lord British invites the Avatar and his companions to his castle. However, in the night of the feast the Guardian attacks again, encasing the castle in a giant gem of blackrock.