Many people have been jumping to conclusions about the new info about the Nuva Cube being the lock to the Bahrag's prison, there is more to it. They are all forgeting something. What about those mysterious pictures of the Nuva Cube in the comics (see the middle of the comic 9, right page, bottom) with the pieces of interlocking rock all around the Cube? This what I think. I think that the Cube is already in existence. It must be hidden somewhere deep underground. The Kal must have access to it.
Then why are the Kal stealing the symbols from the villages?
The reason is Kal need the symbols in each of the villages to use for molding purposes. These symbols (for they have no power as Kopaka says himself) are prefect molds. The indentantions on them are just that: indentations. They do not go all the way through. Most likely the gravity Kal (I don't want to get into that controversy here) will crush a rock into it or something, but that's just details The resulting thing (I'll call it a key) is depicted in the aforementioned picture.
Let me get deeper into what the Nuva Cube is: it is the lock to the prison of the Bahrag and the key to the Toa Nuvas' power. Let me explain. In many of the pictures of the Nuva Cube, you can see energy emitting from the middle of the Cube outward. This energy is the Power of the Nuva. When one of these sides has a "key" in it, the corresponding Toa Nuva loses his power and most likely the other Nuva are strengthened, as depicted by Kopaka's loss of power in comic 9. Once all the "keys" are in place the Nuva Cube probably self-destructs because the power has nowhere to escape and the energy-builtup blows it to pieces. Then the Bahrag are freed. It is doubtful what exactly will happen when the Nuva Cube is destroyed when it comes to its effect on the Toa Nuva.
Well, that's my theory. Tear it to pieces if you feel like it