In Wikipedia https://en.wikipedia.org/wiki/Narayanastra, it is mentioned that Aswathama used Narayanastra against Pandavas.
It also mentions that the owner of Narayanastra is
How did Aswathama get hold of this ?
Here in Drona Parva of Mahabharata Ashwathama himself mentioned that Lord Narayana has given Narayanastra to Drona and him.
Formerly on one occasion, Narayana, assuming the from of a Brahmana, came to my father. Bowing unto him, my father presented his offerings unto him in due form. Taking them himself, the divine Lord offered to give him a boon. My father then solicited that supreme weapon called Narayana. The divine Lord, the foremost of all gods, addressing my sire, said, No man shall ever become thy equal in battle. This weapon, however, O Brahmana, should never be used in haste. It never comes back without effecting the destruction of the foe. I know none whom it may not slay, O lord! Indeed, It would slay even the unslayable. Therefore, it should not be used (without the greatest deliberation). This mighty weapon, O scorcher of foes, should never be hurled upon persons that abandon their cars or weapons in battle, or upon those that seek for quarter or those that wield themselves up. He who seeketh to afflict in battle the unslayable with it, is himself exceedingly afflicted by it! '--My sire thus received that weapon. Then Lord Narayana, addressing myself also, said, 'With the aid of this weapon, thou too shalt pour diverse showers of celestial weapons in battle and blaze with energy in consequence of it. Having said these words, the divine Lord ascended to heaven. Even this is the history of the Narayana weapon which has been obtained by my sire's son. With that I will rout and slay the Pandavas, the Panchalas, the Matsyas, and the Kaikeyas, in battle, like Sachi's lord routing and slaying the Asuras.