The Sword of Absolute Law.
The Lords of Law, one of the two groups of Primal pre-Gods, created a master weapon that embodied their philosophy - the white greatsword Zero-One. When they dlsapeared at the begining of Time, the Primal weapons survived them, hidden as securely as possible.
Zero-One was discovered, and eventually fell into the hands of the cambion Zerhal Avara. He used it to great effect in the Blood War, eventually rising to be one of Bel's greatest generals.
It was stolen from him in the ruins of Dalbin by Sack, and bestowed safely in the Hallows. It was then sealed there by the destruction of the Glarith Key in order to prevent it falling again into the wrong hands.