Narayanastra

[1] This astra ("celestial weapon" in Sanskrit) fires a volley of millions of deadly missiles simultaneously, the intensity of which rises in proportion to the resistance of the target.

[2] The only way to defend against the Narayanastra is, therefore, to show total submission before the missiles hit, which would cause them to stop and spare the target.

Millions of weapons such as chakras, gadas, and ultra-sharp arrows appeared and moved to destroy them; all those who resisted were killed.

Krishna, who knew how to stop the Narayanastra, advised the Pandavas and their army to immediately drop their weapons and utterly surrender to the great astra of Narayana.

[5] When targeted, the Pandava Bhima refused to surrender, considering it an act of cowardice, and attacked the downpour of fiery arrows.

Ashwatthama fires the Narayanastra on Pandavas
After the death of Dronacharya, his son Ashwathama used Narayanashtra against Pandava forces. Lord Krishna advised the Pandavas to immediately drop their weapons and utterly surrender to the great astra of Lord Vishnu to stop the Narayanastra but Bhima refused to surrender. Then Krishna and Arjun began forcibly to take off all weapons from Bhima and to dragg him down from his chariot.
Lord Krishna dragging Bhima from his chariot for saving him from the Narayanastra