After the "Black Egg" lodges itself within the Pyramid, a tube will emerge from the right side of the pyramid.
Fill this tube with 25 zombie souls, staying in vicinity of the tube, so as to let it collect said souls. Once the canister is filled, flip the switch on the wall to the right of the tube (or, left, if you're facing towards the power switch).
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In doing this, the pyramid will open to reveal Samantha, her body frozen and hovering in place - meanwhile, you will then be gifted a 90-second Death Machine power-up. It is at this moment that you'll net the achievement/trophy "Cryogenic Slumber Party", thereby completing Part 1 of the Main Quest. Congratulations!
Proceed to Step 6 if you wish to complete Big Bang Theory.