Nui Stone

"The Nui Stone is active, where a Toa Stone is passive. It doesn’t wait for you to give it your energy – it takes it. Once activated, it will absorb minute amounts of energy from any Toa within a 3000 Kio radius, so slowly and surely that it won’t be noticed until it’s too late. Then that energy can be absorbed by a living being – can you imagine? One being with the power of dozens, maybe hundreds of Toa, or more?"

- Toa Tuyet to Toa Lhikan

The Nui Stone was a powerful one-of-a-kind artifact in the Bionicle universe, with the aggressive tendency to slowly absorb Toa energy from all Toa in a 3000-kio radius. Whoever touched it would gain its power. It glowed red with energy and heat.

Creation
Long ago, the makers of the Nui Stone created it, but knowing its power, decided to destroy it. However, before they could, it was stolen from them and passed around for years.

Tuyet's Home Island
At some point, the Nui Stone ended up on the home island of the Water Toa Tuyet. The Dark Hunters came after it, but they arrived to find that the stone was gone...as was Tuyet. Naturally assuming she had taken the Nui Stone with her to Metru Nui for safekeeping, they went there.

Metru Nui
On Metru Nui, Tuyet hid the Nui Stone in her dwelling and kept it a secret for a long time. Eventually, deciding the Dark Hunters would not stop looking for her if someone did not stop them, she began to murder Matoran, claiming to Toa Lhikan and Nidhiki that it was a 'countdown' by the Dark Hunters, and that she had seven days to give them the Nui Stone. While Dark Hunters were indeed searching for her, they had no idea of such a countdown. Lhikan realized that Tuyet had been the murderer, and used his heat-sensing abilities to locate where she had hidden it.

Tuyet took the Nui Stone back from Lhikan and used it to empower herself, claiming that she would become the 'ultimate solution' and do what the League of Six Kingdoms tried to do. Lhikan and Nidhiki defeated her, however, and Lhikan destroyed it.

The Pit
Millennia later, Toa Mahri Matoro and the Makuta of Metru Nui, in the body of the robot Maxilos, were exploring the ruins of the Pit, looking for the remains of Tuyet, who had been sentenced to the Pit shortly after the incident with the Nui Stone and had died there in the Great Cataclysm.

Makuta ordered Matoro to use his Great Tryna, Mask of Reanimation, to give Tuyet artificial life, claiming that Tuyet could help him recreate the Nui Stone. Matoro protested that she would have no mind when she came back and would not be able to help, but went along with Makuta anyway.

As it turned out, Tuyet had very small fragments of the Nui Stone still embedded in her armor. Makuta revealed that he could recreate the entire Nui Stone from this fragment, given he had another powerful artifact--the Staff of Artakha.

Since the Toa Nuva are in the process of retrieving the Staff, but will apparently be beat to it by another Makuta, it is likely that the Makuta of Metru Nui will end up with the Staff and recreate the Nui Stone.

Trivia

 * The Nui Stone was first revealed in the BZPower-exclusive short story The Many Deaths of Toa Tuyet.