The Null Sword is a powerful weapon created by the Avatars. It is made of a substance similar to dark matter where a containment field forms it into the shape of a sword. When it comes into direct contact with any sort of matter or energy, it completely obliterates it. As such, it has the capacity to kill gods in a single strike.
The Null Sword however is incredibly dangerous to use. Collateral damage is frequent. And if two Null Swords ever cross blades, it creates a dark matter supernova that can obliterate entire star systems. It was for this reason why all but one of the Null Swords were destroyed.
The last Null Sword was in the hands of an Avatar named Durana. It was wielded by him until Savien Wesiford defeated him. From there, an Avatar assassin named Reaper acquired the sword to use it for vigilante purposes.