[TheCrimsomBlade]

You don't have stand anyone in front of the door!
send your party into the hallway and then south through the door into the crypt. Do in all the Shadows and then look in the northwest corner and find a casket that is a secret door then open it.
Next make sure the hallway on the northwest is clear of anyone and then open the door to the mirror room. Everybody get clear of the doorway and send one party member back to the prism room to turn the prism to the south and then have him throw the switch that fires the lighting bolt. The Bolt will go through the crypt and into the mirror room and unlocking the door to the treasury.

one other tip. Look at every fallen body from this point on save any keys you find you'll need them soon.