first you have to find 3 sets of keys (hate, destruction, and terror from the summoner in act2, nihilatak from act5, and the countess from act1 respectively)
then you must open a portal to a random place (you must transmute 1 set of keys in act5) where you will fight either uber duriel, lilith, or uber izual.
after you find all three body parts (each one of them will drop a different part, such as "baal's eye") you must put them all into the cube and transmute.
when you go inside the portal, you will be transported into tristram where you will fight the three ubers (diablo, baal, mephisto)
