When you reach the portal room for the second time, when you have to
switch the time to the present for the first time, the Prince will say
that something is wrong. You must run the switches on the wall to make
the portal work correctly. Do not run the switches on the wall. Instead,
enter the portal as it is and you will switch the time to the present.
Save the game, then exit and load the game. You will now be playing as
Sandwraith. In this mode you will constantly lose life and much more
health when hit than in normal mode. However your attacks will be very
powerful and you are much more quicker. 
Note: Save the game on a different file if you wish to continue the 
game as the Prince.

Do the same thing as instructed above, but do not exit the game.
Instead, continue playing until you die, or just jump into a pit. Either
way, you must die a few times for this to work. Eventually when you
retry and come back to life, the prince's body will become completely
invisible except for his swords and his head. Note: You only have one
chance to play the game in this form; if you die again and retry you
will come back as the sandwraith and will not be able to do it again
unless you start a new game.