Narayanastra is the personal missile weapon of Vishnu in his Narayana form, this astra lets loose a powerful tirade of millions of deadly missiles simultaneously.
The Narayanastra (IAST: nārāyaṇastra)(or Narainastra) was the personal weapon of lord Vishnu in his Narayana(or Naraina) form. This astra ("weapon" in Sanskrit) in turn fires a powerful tirade of millions of deadly missiles simultaneously. The intensity of the shower increases with increase in resistance. The only way of defense towards this missile, is to show total submission before the missiles hit. This in turn will cause this weapon to stop and spare the target.
Ashwathama, a Kuru warrior-hero in the epic Mahabharata unleashes this weapon on the Pandava forces. Lord Krishna, who is an Avatar of Vishnu tells the Pandavas and their warriors to drop their weapons and lie down on the ground, so that they all surrender completely to the power of the weapon. It was also said that this weapon can be used only once in a war and if one tries to use it twice, then it would devour the user's own army.
When it was used, Ekadasha (Eleven) Rudras appeared in the sky to destroy Pandavas. Millions of types of weapons like Chakra, Gadha, ultra sharp arrows appeared in rage to destroy them.Who ever tried to offend were destroyed. Shri Krishna who knew how to cool down the Narayanastra advised Pandavas and their army to immediately drop all types and sorts of weapons from their hands and utterly surrender to the great astra of Lord Vishnu. Everybody does the same and survives.
When targeted, the Pandava hero Bhima refuses to surrender thinking that it a cowardice act, and attacks the downpour of fiery arrows. The Narayana weapon concentrates its shower on him, and he gets steadily exhausted. However, he was not killed as Krishna and his brothers restrain him at the right time.