This is rewarded for finishing the adventurer's puzzle.
Quick step guide through the maze (beware of spoilers!) (without getting all the artworks! This is just the solution of the puzzle):
- The adventurer's special ability lets her jump over those spikes. Use this to get to the pyramid and cross it in its entirety (don't worry about your companions, they will come along)
- Enter the pyramid, again with your special abilities. You still won't have to worry about your companions.
- Have the adventurer go down the ladder that goes down the most and recover the Ankh
- Carry this Ankh to the top most ladder, use your special ability to hop over the spikes. Insert the Ankh in its rightful place, go on and step on the swith with the rabbit.
- Saw the place which opened now? Have your two other companions jump into this little cave.
- Now you will have to always match the right symbols. The two companions have to get on the right switch which is shown by the adventurer on the walls. Note that sometimes you must only stand on one switch (the other character has to stand on nothing). Carry on until you reach the Sarcophagus.
- Push the sarcophagus over the edge and then to the left onto the switch.
- Now to get the sarcophagus all the way down, you have to be quick and match the right symbols with one character. First, stand with your character on the blue switch (I believe it's that one first).
Step on the switch left of the Sarcophagus left with the adventurer and after it's passed the gate, jump over the gap right down and sprint to the left, over the gap again and on the switch again, so that the Sarcophagus can smoothly go on one more level down. Now jump over that gap again, quickly switch to your other character, have him step on the violet switch (I believe that one's second), again sprint to the left with the adventurer, jump the gap and onto the switch to get the Sarcophagus to the lowest point. If you failed - no problem, just go up again and repeat.
- Okay, onto the last part... Move the Sarcophagus all the way to the right (killing the other guy in the process) and you will fall down the collapsed bridge.
- Again, you have to match symbols. Look at the three symbols (there is a feather and some other things combined with the two animal-symbols). Match the right symbol with the adventurer (stepping on the switches on the left) with the right animal-symbols with your companions and a green light will show. Do this for each symbol (three times) and the gate for your two companions will open and a box will be at your disposal.
- Have one of your companions drop the box down to the adventurer.
- Have the adventurer pull the box onto the switch so that the appearing ladder won't disappear again.
- Have all your three characters step on the final switch on the right and the door will open...
Voilà - puzzle solved.