THIRD CHALLENGE TOMB (Waterfall of Sobek)
CHALLENGE TOMB ENTRANCE: The entrance to this Challenge Tomb is located in the northeast corner of the Waterfall of Sobek section of the Overworld. (screenshot) Its location is marked with a star on the Overworld map. (screenshot)
SINGLE-PLAYER CHALLENGE: Enter the tomb, smash some vases for gems, and scope out the puzzle. Amid various clusters of spikes there are 9 retracting mirrors and 3 pressure pads that control the mirrors. Each pad raises some mirrors and lowers others. In addition to Lara's weight, you have one metal ball to work with. (screenshot)
The goal is to depress the correct pads and shine a beam of light from the Staff of Osiris that will pass through, and thus destroy, the 3 Marks of Set.
There may be other possible solutions, but here's what worked for me: Roll the metal ball onto the pad in the back left corner, near the shimmering gate. Then have Lara stand on the pad in the near right corner and face left. When she shines the beam, it should pass through the first Mark, reflect off the mirror in the left corner, then the mirror near the metal ball, go through the Mark farthest from the entrance, reflect off one of the 2 mirrors in the top right corner, then the one between it and Lara, and finally reflect straight across the room to strike the third Mark. (screenshot)
2-PLAYER CO-OP CHALLENGE: You don't have a metal ball, so stand Lara on the pressure pad near the shimmering gate. Position Isis or Horus on the pad in the near right corner and direct the beam just as in the single-player puzzle. (screenshot)
3- or 4-PLAYER CO-OP CHALLENGE: In this slightly more complicated version of the puzzle described above, the Mark of Set nearest the entrance is sitting on top of a pillar marked with Osiris's Glyphs. (screenshot) So in addition to arranging the mirrors correctly, one of the Egyptian gods must hold the staff aloft to keep the column retracted, while the other shines the beam of light to destroy the Marks.
When you have destroyed all 3 Marks, the shimmering gate comes down so you can claim your rewards: the AUTOMATIC SHOTGUN and a MAX AMMO UPGRADE. (screenshot) Destroy the sarcophagus and pick up the gems before returning to the Overworld.
†UPDATE HISTORY: 12/30/14 - First draft of walkthrough posted.
ACKNOWLEDGEMENTS: Thanks to Matt T. for input on the 3-player version of this challenge, which I still haven't attempted.
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