Nuva Cube

The Nuva Cube was a lock, specifically to the Bahrag queens' prison which they were trapped in by the Toa Mata. When the Toa unleashed an experimental attack which came with the side effect of plunging them into Energized Protodermis, causing them to evolve into stronger Toa called Toa Nuva.

It can be unlocked when all six Nuva Symbols latch into their respective slots. When the symbols were stolen from their respective Suvas, the Toa Nuva lost their elemental powers and their Kanohi Nuva turned a grey color, however, they could still use their mask powers.

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