Nuva Symbols

The Nuva Symbols were a set of mysterious and unique stone carvings which contained the Toa Nuva's Elemental Powers. If these symbols fell in the hands of the Toa's enemy, the Toa lost his or her powers. The Nuva Symbols served as keys for the Nuva Cube, a lock to the Bahrag's prison. The Bohrok-Kal exploited this in an attempt to free the Bahrag queens. The Toa Nuva also had a mental connection to the symbols, allowing them to pour their energy into the Bohrok-Kal, thus overcharging them.

After the defeat of the Bohrok-Kal, the Turaga and Matoran hid the Nuva Symbols, so that future enemies (such as the Rahkshi) would not be able to find them. During the destruction of Ta-Koro, Tahu's Nuva Symbol was rescued and preserved, allowing him to maintain his Fire powers. The Matoran of Mata Nui took the symbols with them when they migrated back to Metru Nui, and later when they evacuated the Great Spirit's body onto Spherus Magna.

Trivia

 * While the Nuva Symbols were made in Artakha, the comics originally stated that each one was found or created somewhere in Mata Nui.
 * While the Nuva Symbols did not reappear in the 2015 BIONICLE reboot, their likenesses appeared on each Toa's armor.