Nuva Cube

The Nuva Cube was a key, specifically to open up the Bahrag sisters prison which they were trapped in by the Toa Mata when they unleashed an experimental attack which came with the side effect of plunging them into liquid protodermis, causing them to mutate into stronger toa called Toa Nuva. It is made of combining all six Nuva symbols. When the symbols were stolen from their tempals which guarded them, the Toa Nuva lost their elemental powers and turned a grey colour. However, they could still use their mask powers.

The Bohrok Kal almost assembled it once, but the Toa Nuva managed to stop them before they could.