Nagapasha or Nagastra is a powerful arrow capable of huge destruction

There is a story about the origin of Nagastra in the Kamba Ramayana. It is said that Nagapasha was in the beginning a very powerful Snake. The demon Nagasura was created by Brahma. The main weapon was a piercing cry. Many pregnant women, children and old people were killed when the Snake cried on reaching the kingdom of Nagasura. Nagasura's entire army was killed by Nagapasha and he was the one who died. Vishnu and Shiva gave blessings to Nagapasha for killing the demon. Nagapasha was one of Shiva's ornaments. She wanted to go on a tour of the world.

Shiva told him not to visit the island. The snake didn't listen to his advice and went to the island. The Nagas on this island were attacked frequently. The snake could not survive the attack by Garuda. Shiva was where Nagapasha took refuge. Shiva said that Nagapasha won't interfere with Garuda from now onwards. The arrow was created from that day. Through austerities, the demons, Devas and Humans pleased Shiva, Brahma and Vishnu. In the epics, Nagastra was used. Thousands of snakes came down from the sky when Nagastra was used. It did emit poison in some cases. 

Nagastra in Ramayana

Nagastra was used by Ravana against Sri Ram

Thousands of snakes darted towards Sri Ram. The arrows had mouths that looked like serpents. Sri Ram wanted to stop the Nagastra. The arrows became Garudas and the serpent arrows were cut off. The enemy of serpents is Garuda. The effect of Nagastra was nullified by Garuda. Indrajit used it against Lakshman. This made him dumb. 

Karna had used it against Arjuna

 Kunti was told by Karna that he wouldn't use the Nagastra twice against Arjuna. The Nagastra hit the crown of Arjuna when Karna used it the first time.