In the beginning of the universe brahma created a multitude of beings, of which some asuras always defied his rules and caused havok.
To destroy them, Brahma and the other sages conducted a thousand-year sacrifice, from which a divine being emerged out of the sacred fire. This being was then transformed into a mighty battle sword and given to Rudra, who used it to wipe out the asuras that were wreaking havok. The very fact that such a prolonged sacrifice was needed to forge this weapon shows that no existing weapon was powerful enough to accomplish the task.
And it looked exceedingly beautiful in consequence of the sacrificial plates and vessels all made of gold. All the foremost ones among the gods took their seats on it. The platform was further adorned with Sadasyas all of whom were high regenerate Rishis. I have heard from the.Rishis that soon something very awful occurred in that sacrifice.
It is heard that a creature sprang (from the sacrificial fire) scattering the flames around him, and whose splendour equalled that of the Moon himself when he rises in the firmament spangled with stars. His complexion was dark like that of.the petals of the blue lotus. His teeth were keen. His stomach was lean. His stature was tall. He seemed to be irresistible and possessed of exceeding energy. Upon the appearance of that being, the earth trembled. The Ocean became agitated with high billows and awful eddies.
Meteors foreboding great disasters shot through the sky. The branches of trees began to fall down. All the points of the compass became unquiet. Inauspicious winds began to blow. All creatures began to quake with fear every moment. Beholding that awful agitation of the universe and that Being sprung from the sacrificial fire, the Grandsire said these words unto the great Rishis, the gods, and the Gandharvas.
This Being was thought of by me. Possessed of great energy, his name is Asi (sword or scimitar). For the protection of the world and the destruction of the enemies of the gods, I have created him. That being then, abandoning the form he had first assumed, took the shape of a sword of great splendour, highly polished, sharp-edged, risen like the all-destructive Being at the end of the Yuga.
Then Brahman made over that sharp weapon to the blue-throated Rudra who has for the device on his banner the foremost of bulls, for enabling him to put down irreligion and sin. At this, the divine Rudra of immeasurable soul, praised by the great Rishis, took up that sword and assumed a different shape. Putting forth four arms, he became so tall that though standing on the earth he touched the very sun with his head.
Meaning this would be easily most powerful weapon. In Mahabharat era the sword was wielded by Drona, kripa and Nakula.