I've defeated the three priestesse and sherincal, asked for nickadaemus's help, but when I rotate the mirror west and fire the prism north the lightning bolt hits the door and does nothing.

Help?

There are some other posts that deal with this, but here goes:
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Very direct spoiler...
Use the painting on the first floor to transport into the room with the shadow door (use the Lysan option to get into the shadow room). Find the hidden exit door, you cannot open it at this time, now try to open the shadow door and get hit with the unholy blight effects. Now open the door to the hallway and move your characters out of the shadow room. Go back to the prism (make sure the mirror outside the northern door is rotated to the left) and fire it through the northern door. The lightining bolt should hit the shadow door and disable the trap allowing you to access the alter room so that you can banish Caged Fury...