How to get the Megasphere on level 29, Twilight Descends:

The Megasphere rests on the top edge of a long wall at the
southeast quadrant of the map.  Here's how to get it:

At the northeast corner of the map, look for a long tunnel
that runs west to east, then north to south.
At the south end of the tunnel is a platform, or pier, that
extends south into shallow water.  Just past the south edge
of the pier is a square teleporter pad in the water.

Below is a diagram that approximates the appearance of the
area on your Automap:


     |__|
    ||  ||
    ||  ||
    ||__||  <-- South
  ___/__|___    end of
 /  |   |   \   tunnel
|   |   |    |
    |___|
     |_|  <-- Teleporter
              at end of
              pier/platform


If you step on the teleporter by crossing its north edge
(stepping south off the pier) you'll materialize on the top
edge of a wall that's located at the south end of the map.
Walk east along the wall to the Megasphere.
If you step onto the teleport pad by crossing any other
edge, you'll appear in the darkened, elevated room which is
located just southwest of the west end of the tunnel.