Hourglass

This will unlock after the first batch of puzzles that introduce you to the game mechanics. They start off quite simple, you use a portal to make copies of yourself to use to move blocks or stand or pressure plates.

A great walkthrough for the whole game can be found in the "Prophecy" solution found here:

This will pop after the cutscene in the pyramid, exiting out, pushing the button on the walkway for the scarab path and walking partway down the path.