The Box of Infinite Complexity code is a randomly-generated 20-digit number that is written on a scrap of paper in the War Room in Kaptar. It can be entered into the Box of Infinite Complexity in Crankshaft cavern to unhook the box from the retractable stairway. If the box is in the air when the code is entered, it will fall to the ground, granting the Sweet Revenge achievement.
An example of random code
The fallen box