I found it in Edron here you go down above a graveyard:
And then click on the tomb
Then the tomb opens and you get the cube! :D
Note: The missing part of the cube can only be found after defeating The Last Planegazer.
If you did not kill him before trying to open the grave, you will not find anything (The message says there's still something hidden, though).
This is a clue for the next step on this mysterious quest
Next clue appears on cobra bastion , some messages appears when you are on that zone and then as an Ilussion it appears a complete cube for 1 second.