Mirror crystal causes strange twists and turns in space.
Effect: A creature standing on mirror crystal can look down and see all the other mirror crystal spaces within 20 squares. Creatures can make ranged attacks through mirror crystal, targeting any creature on or adjacent to another square of mirror crystal. The range to a creature attacked through mirror crystal is 1 square.
Usage: Start an encounter with artillery monsters hidden from sight but positioned on squares of mirror crystal. Then watch as the players figure out how to return the monsters’ fire!
Published in Dungeon Master's Kit, page(s) 211, Dungeon Master's Guide, page(s) 68.