Brahmastra has been mentioned quite frequently in ancient texts and Mahabharat. As described in the shastras, a Brahmastra is an astra which was created by Lord Brahma. It is sometimes known as the Brahma Astra (missile weapon). As mentioned in the number of Puranas, Brahmastra is considered to very deadliest of weapons. When Brahmastra is discharged, neither a counterattack nor a defence of any kind can stop it.
This weapon can only be neutralized by Brahmadanda. Karna had the knowledge of this potent astra. He was not able to use this because of the curse of Parshuram. Vishwamitra had used it against Vashishtha. However, Vashishtha had neutralized this weapon and defeated Vishwamitra.