"That sword doesn't seem to belong to you though." Asura Emperor felt the horrific aura released by the sword. Not only he was astonished but also had some fear in his heart. He saw the cold and pretending smirk in Johnny's face, so he realized everything.
"Since I am the one holding this sword, it is mine. Don't tell me, the great Asura Emperor got scared by the aura of this sword." Johnny has only one goal, and that is to make Asura Emperor accept this bet. This is why Johnny was willing to release a one-time sword aura against the Asura Emperor.
This sword was nothing great in particular, except its ability to absorb soul. But since it could absorb other's souls, it could store some of the soul pressure from the soul that was absorbed. Later, that soul pressure could be released from it.