"Your first mistake was attacking my friends, your second mistake was underestimating my power, both will cost you your life." Erebus to Ballom
After Erebus became a Toa Nuva, he was granted adaptive armor, currently in its Combat form, giving him increased maneuverability, defense, and strength. Never really liking the bulky projectile weapons Erebus abandoned it and decided to keep the swords created by Metara. Erebus traveled back to his homeland of Whiro with over a hundred matoran of Artakha and Toa Nuva Metara. Erebus and the group entered the mines of Whiro, and cleared the island of the rarest metal in the universe, Black Protosteel.The Matoran of Whiro had no problem with this because they knew that if the Brotherhood of Makuta had discovered the metal a war could erupt.
Erebus Nuva and his team of Toa are servants of the Order of Mata Nui and are an elite group of Toa, despite the fact that they haven't been around as long as the original Toa Nuva. Their adaptive armor shows the elite status. Erebus wears the one of a kind mask the Kanohi Zaethos Nuva, the mask of mimicry. It allows Erebus to copy the mask power of both friend and foe and amplify it to Nuva Levels.
Erebus brought the precious metal back to Artakha where the metal was stored away in case it would ever be needed. Because of the swords composition Erebus' Elemental Power can be amplified through them. Both Swords have the symbol of the three virtues on the blades, and a Kanohi Hau on the hilt. Later they were given to Metara Nuva, Toa of Iron, where he removed the black protosteel core and filled it with Golden protosteel. The Swords were then presented to Hyperion Nuva: Toa of Light, as his new weapons
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Credit goes to Heir of the Chronicler on Bzpower for the swords and other custom bits. Couldn't have done it without you Tukua