The mysterious box was a roughly triangular-shaped box that was created by the Rakata of the Infinite Empire. It was used as a prison for the mind; it kept the mind trapped inside a vast, unending whiteness broken only by a group of Rakatan pillars in the center.
The mysterious box had been used to keep only the most dangerous of criminals until the fall of the Empire, when it was lost until certain individuals discovered it and discovered its unique use. Revan came across this box on Korriban with instructions to deliver it to Motta the Hutt on Tatooine. The Rodian who gave the box to Revan gave him explicit instructions not to open the box.
At this point, the player has a choice, either to deliver the box or open it. If the box is delivered, nothing happens. However, if the box is opened, Revan's mind is taken into the prison, where he meets a Rakatan criminal. The criminal challenges him to a game of riddles, the winner of which would be allowed to go free in Revan's body. Revan ultimately won and returned to deliver the box.
- For information on this subject beyond The Old Republic, see: Mind prison on Wookieepedia