How to get the blue key and teleporting Soul Sphere in Doom
64 level 15, Dark Entries:

(Note: For a video demonstration of the walkthrough below,
click here.)

Background: The blue key is at one end of an isolated
square room; At the opposite side of the room is a switch
on the wall.  When the switch is Activated, a Soul Sphere
materializes, and then seems to teleport randomly between 5
pedestals arranged along one of the other walls.  After the
Soul Sphere has teleported several times, 5 thin slabs
appear along the opposite wall, and on the back of each
slab is a switch.

First, Activate the single switch on the wall opposite the
blue key, then quickly turn to look at the 5 pedestals.
Look for the pedestal where the Soul Sphere FIRST appears.

After the Soul Sphere has stopped teleporting, go over to
the switch on the slab that is directly across from the
pedestal where the Soul Sphere first appeared.  Activate it
to lower the pedestal.

If you chose the correct switch, the Soul Sphere will be
sitting on the lowered pedestal, and you can pick it up.
(If you chose the wrong switch, an armor bonus will be on
the lowered pedestal.  Picking it up will reset the switch
opposite the blue key, and you can try again for the Soul
Sphere.)

After you pick up the Soul Sphere, grab the blue key.