Narayana Weapon

Formerly on one occasion, Narayana, assuming the from of a Brahmana, came to my father Drona. 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.

1. It never comes back without effecting the destruction of the foe. 
2. I know none whom it may not slay, 
3. 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

1. persons that abandon their cars or weapons in battle, or 
2. upon those that seek for quarter or 
3. 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.

Meanwhile, Drona's son, having said those words, O Bharata, touched water and invoked the celestial weapon called the Narayana.'"

'When the weapon called Narayana was invoked,

1. violent winds began to blow with showers of rain, and 
2. peals of thunder were heard although the sky was cloudless. 
3. The earth trembled, and the seas swelled up in agitation. 
4. The rivers began to run in a contrary course. 
5. The summits of mountains, began to split. 
6. Diverse animals began to pass by the left side of the Pandavas. 
7. Darkness set in, the sun became obscure. 
8. Diverse kinds of carnivorous creatures began to alight on the field in joy. 

The gods, the Danavas, and the Gandharvas, O monarch, all became inspired with fear. Beholding that tremendous agitation (in nature), all began to ask one another loudly about its cause. Indeed, seeing that fierce and terrible weapon invoked by Drona's son, all the kings, inspired with fear, felt great pain.'

http://www.sacred-texts.com/hin/m07/m07192.htm

Then

1. thousands of arrows with blazing mouths appeared in the welkin,
2. resembling snakes of fiery mouths, 
3. like the very rays of the sun in a moment shrouded all the points of the compass, the welkin, and the troops. 
4. Innumerable iron balls appeared, like resplendent luminaries in the clear firmament. 
5. Sataghnis, some equipped with four and some with two wheels, and 
6. innumerable maces, and 
7. discs, with edges sharp as razor and resplendent like the sun, also appeared there. 

http://www.sacred-texts.com/hin/m07/m07196.htm

Kesava quickly forbidding the troops, by motion of his arms, to fly away said these words, Speedily lay down your weapons, all of you, and alight from your vehicles. Even this is the means ordained by the illustrious one, (viz., Narayana himself) for baffling this weapon. Come down on the earth, all of you from your elephants and steeds and cars.

If you stand weaponless on the earth, this weapon will not slay you. In those places where you will fight for quelling the force of this weapon the Kauravas will become more powerful than you. Those men, however, that will throw down their weapons and alight from their vehicles, will not in this battle, be slain by this weapon. They, however, that will, even in imagination, contend against this weapon, will all be slain even if they seek refuge deep beneath the earth'. The warriors of the Pandava army, hearing, O Bharata, these words of Vasudeva, threw their weapons and drove away from their hearts all desire of battle.