This quest is either not finished yet (cip been working on it in each update), or if someone finished it, he/she did not come forward and made it public.
No one even knows yet on how to create the supreme cube (as far as I know). So, unfortunately there is no way to know the use of the Cube by now. We will definitely know in the future, tho.
The last part of the quest discovered was a relation between Scarlett Etzel, the Cube and the Cobra Amulet (which can be opened if inside a secret room). After that, no one really knows whats the next step is nor if opening the amulet is actually necessary. Would suck tho, to depend on a 60kk item to complete a quest :l